The words ending with the sound “o” (“-eau”) are generally masculine.
Exemple: un château.

I think the most useful rule of thumbs is that words ending with -e or -tion are usually feminine, others are usually masculine. I don’t know if someone can come with precise figures, but I would say this holds for more than 80% of all words.

A lengthy but more refined version.

Même pour un francophone, le genre d’un mot n’est pas toujours évident, et de nombreux jeux de société ont des questions sur le genre des mots, surtout s’ils sont faiblement usités…

Pour mémoriser, j’accole systématiquement un adjectif (dont on peut reconnaître le genre à l’oreille) qui complète la signification, quitte à faire un pléonasme pour en confirmer le sens :

• un grand apogée
• un interstice tout petit
• …

edit remplacement de minuscule par tout petit

Yes; rules exist, but they predict gender mostly with at least 80% (but not 100%) accuracy.

See

• Predictability in French gender attribution:
  A corpus analysis by Roy Lyster: https://escholarship.mcgill.ca/downloads/0c483p62x?locale=en

I quote from the last paragraph (from p 22 of the PDF of 24 pages above) which answers your question more optimistically:

  Gender attribution rules based on noun endings, given their reliability and
systematicity, are worthy of more attention in French reference books and French
L2
classrooms. The foregoing corpus-based study confirmed that predictive rules for
gender attribution do exist and apply to as many as
80
per cent of the nearly
10,000
nouns included in the analysis. More importantly, classroom studies have demonstrated that gender attribution rules are both teachable and learnable. Regardless of
age, L2
learners can benefit from form-focused instructional activities that promote
awareness of gender attribution rules and that provide opportunities for practice
in associating grammatical gender with orthographic representations of constituent
rhymes of literally thousands of nouns—both animate and inanimate alike.

• Adult L2 Acquisition of French
  Grammatical Gender:
  investigating
  sensitivity to phonological and
  morphological gender cues:
  https://www.cs.cmu.edu/~csisso/docs/honoursThesis.pdf.

I chanced on the following from Reddit:

I learned Spanish first and then French, and Spanish’s gender is a lot more transparent for a novice. In Spanish, generally words ending in -o are masculine and those ending in -a are feminine. In French, generally words ending in a vowel SOUND are masculine and those ending in a consonant SOUND are feminine. The fact that a word like ‘garçon’ ends in a consonant letter but a vowel sound may be confusing for a new language learner.

With that said, there are a lot of times when French helps me figure out a gender in Spanish. For example, ‘fuente’ and ‘puente’ are ambiguous in Spanish, but in French, the equivalent cognates ‘fonte’ and ‘pont’ are pretty transparent.

There is no general rule to determine the gender of a noun based on its spelling, however, many endings are either deterministic or at least strong clues to know the gender.

Here are some statistics based on a list of French words extracted from the Lexique 3.80 database which contains words built from 64.7 millions words found in 218 books from 1950 to 2000 and subtitles from 9474 movies, including TV series. Only singular (genre=f) common names (cgram=NOM) are analyzed. Results are not weighed by usage frequency.

Out of 27060 common names, 15292 are masculine (56.51%) and 11768 are feminine.

The following curve is displaying the distributions of noun endings usage according to their gender probability. All endings used by at least ten nouns are considered. The leftmost value shows that there are about 19000 nouns/endings combination sharing all endings having a probability between 95% and 100% to be feminine while the rightmost value shows that there are almost 32000 nouns/endings combination sharing all endings having a probability between 95% to 100% to be masculine. Note that most words are counted multiple times as endings from one to five characters are taken into account.

After analyzing the most frequent endings, i.e. those appearing in more than one hundred words, I get these numbers:

411 (96.028 %) out of the 428 words ending in -ette are indeed feminine.

Here are the most frequently feminine endings (> 90%):

ending | percent | total| masc. |  fem.|
trice  | 100.000 |  152 |     0 |   152|
logie  | 100.000 |  138 |     0 |   138|
ienne  | 100.000 |  137 |     0 |   137|
hie    | 100.000 |  109 |     0 |   109|
euse   | 100.000 |  473 |     0 |   473|
erie   | 100.000 |  349 |     0 |   349|
ction  | 100.000 |  152 |     0 |   152|
ance   | 100.000 |  238 |     0 |   238|
alité  | 100.000 |  122 |     0 |   122|
ation  |  99.910 | 1107 |     1 |  1106|
tion   |  99.865 | 1486 |     2 |  1484|
rie    |  99.771 |  436 |     1 |   435|
lité   |  99.638 |  276 |     1 |   275|
ilité  |  99.306 |  144 |     1 |   143|
ture   |  99.254 |  134 |     1 |   133|
use    |  99.229 |  519 |     4 |   515|
mie    |  99.213 |  127 |     1 |   126|
ence   |  99.043 |  209 |     2 |   207|
sion   |  98.953 |  191 |     2 |   189|
enne   |  98.788 |  165 |     2 |   163|
rice   |  98.734 |  158 |     2 |   156|
nne    |  98.723 |  235 |     3 |   232|
ogie   |  98.601 |  143 |     2 |   141|
nce    |  98.495 |  465 |     7 |   458|
esse   |  98.291 |  117 |     2 |   115|
ise    |  98.137 |  161 |     3 |   158|
gie    |  97.838 |  185 |     4 |   181|
ie     |  97.561 | 1353 |    33 |  1320|
ité    |  97.540 |  691 |    17 |   674|
ière   |  97.482 |  278 |     7 |   271|
nie    |  97.059 |  102 |     3 |    99|
ion    |  96.544 | 1765 |    61 |  1704|
ette   |  96.028 |  428 |    17 |   411|
tte    |  95.155 |  516 |    25 |   491|
ante   |  95.105 |  143 |     7 |   136|
ée     |  94.515 |  474 |    26 |   448|
se     |  94.463 | 1210 |    67 |  1143|
elle   |  94.186 |  172 |    10 |   162|
ine    |  93.800 |  500 |    31 |   469|
nte    |  92.827 |  237 |    17 |   220|
ose    |  91.667 |  108 |     9 |    99|
ade    |  91.176 |  170 |    15 |   155|
ce     |  91.165 |  747 |    66 |   681|
té     |  90.665 |  857 |    80 |   777|

Here are the most frequent masculine endings:

ending | percent | total| masc. |  fem.|
tage   | 100.000 |  139 |   139 |     0|
sme    | 100.000 |  519 |   519 |     0|
rd     | 100.000 |  265 |   265 |     0|
lisme  | 100.000 |  100 |   100 |     0|
isme   | 100.000 |  486 |   486 |     0|
ien    | 100.000 |  209 |   209 |     0|
ement  | 100.000 |  923 |   923 |     0|
ateur  | 100.000 |  250 |   250 |     0|
ard    | 100.000 |  237 |   237 |     0|
ment   |  99.899 |  986 |   985 |     1|
ent    |  99.726 | 1095 |  1092 |     3|
ier    |  99.629 |  539 |   537 |     2|
nt     |  99.464 | 1493 |  1485 |     8|
al     |  99.429 |  175 |   174 |     1|
er     |  99.401 |  835 |   830 |     5|
k      |  99.138 |  116 |   115 |     1|
ing    |  99.099 |  111 |   110 |     1|
at     |  99.038 |  208 |   206 |     2|
d      |  99.031 |  413 |   409 |     4|
en     |  98.913 |  276 |   273 |     3|
ant    |  98.895 |  362 |   358 |     4|
um     |  98.857 |  175 |   173 |     2|
age    |  98.849 |  869 |   859 |    10|
et     |  98.575 |  351 |   346 |     5|
t      |  98.545 | 2681 |  2642 |    39|
teur   |  98.381 |  556 |   547 |     9|
nage   |  98.165 |  109 |   107 |     2|
lage   |  97.945 |  146 |   143 |     3|
ng     |  97.902 |  143 |   140 |     3|
oir    |  97.761 |  134 |   131 |     3|
seur   |  97.727 |  176 |   172 |     4|
in     |  97.723 |  483 |   472 |    11|
lon    |  97.674 |  129 |   126 |     3|
c      |  97.590 |  166 |   162 |     4|
ton    |  97.500 |  120 |   117 |     3|
ot     |  97.487 |  199 |   194 |     5|
l      |  97.462 |  591 |   576 |    15|
au     |  97.368 |  228 |   222 |     6|
an     |  97.333 |  225 |   219 |     6|
g      |  97.238 |  181 |   176 |     5|
rage   |  97.059 |  102 |    99 |     3|
el     |  96.970 |  132 |   128 |     4|
il     |  96.825 |  126 |   122 |     4|
if     |  96.721 |  122 |   118 |     4|
u      |  96.266 |  482 |   464 |    18|
ain    |  96.154 |  104 |   100 |     4|
ou     |  96.078 |  102 |    98 |     4|
eau    |  96.020 |  201 |   193 |     8|
r      |  95.876 | 2716 |  2604 |   112|
ir     |  95.767 |  189 |   181 |     8|
m      |  95.455 |  286 |   273 |    13|
it     |  95.420 |  131 |   125 |     6|
rt     |  94.118 |  102 |    96 |     6|
eur    |  93.989 | 1464 |  1376 |    88|
ur     |  93.951 | 1521 |  1429 |    92|
i      |  93.883 |  376 |   353 |    23|
h      |  93.860 |  114 |   107 |     7|
f      |  93.720 |  207 |   194 |    13|
leur   |  93.103 |  145 |   135 |    10|
ge     |  92.545 | 1006 |   931 |    75|
o      |  90.094 |  424 |   382 |    42|

And here are the endings less likely to help figuring out the word genders:

ending | percent | total| masc. |  fem.|
ire    |  69.068 |  236 |   163 |    73|
a      |  57.570 |  568 |   327 |   241|
n      |  48.993 | 3525 |  1727 |  1798|
son    |  48.696 |  115 |    56 |    59|
pe     |  45.856 |  181 |    83 |    98|
re     |  43.318 | 1519 |   658 |   861|
é      |  40.311 | 1352 |   545 |   807|
que    |  39.093 |  353 |   138 |   215|
ule    |  37.209 |  129 |    48 |    81|
ue     |  33.849 |  517 |   175 |   342|
ique   |  33.679 |  193 |    65 |   128|
le     |  32.061 | 1048 |   336 |   712|
e      |  31.672 |12557 |  3977 |  8580|
he     |  30.420 |  286 |    87 |   199|

If we only analyze the gender based on the last character of each word, we can see it is easier to spot mostly masculine endings while this is not the case with feminine words. Only -e (with accented variants) and -n endings are more likely to be used by feminine words but not that much, all other ones are more likely to be masculine, including -a.

term.  | percent | total| masc. |  fem.|

v      | 100.000 |    3 |     3 |     0|

q      | 100.000 |    3 |     3 |     0|

j      | 100.000 |    2 |     2 |     0|

.      | 100.000 |    3 |     3 |     0|

'      | 100.000 |    3 |     3 |     0|

k      |  99.138 |  116 |   115 |     1|

d      |  99.031 |  413 |   409 |     4|

t      |  98.545 | 2681 |  2642 |    39|

c      |  97.590 |  166 |   162 |     4|

l      |  97.445 |  587 |   572 |    15|

g      |  97.238 |  181 |   176 |     5|

u      |  96.266 |  482 |   464 |    18|

r      |  95.876 | 2716 |  2604 |   112|

m      |  95.455 |  286 |   273 |    13|

i      |  93.883 |  376 |   353 |    23|

h      |  93.860 |  114 |   107 |     7|

f      |  93.596 |  203 |   190 |    13|

p      |  90.698 |   86 |    78 |     8|

o      |  90.094 |  424 |   382 |    42|

b      |  89.130 |   46 |    41 |     5|

x      |  87.500 |    8 |     7 |     1|

z      |  85.714 |    7 |     6