When one a verb follows another verb, the second one must be in infinitive form.

Je peux savoir…=>Comment puis-je savoir…

[Note it is more to common to see comment est-ce que je peux savoir]

Infinitive governed by a verb

In fact, it is the same as in English. you would say

"How does he know?" and not "how does he knows?

The second verb is not related to the subject but to the first verb, so it takes the infinitive form whatever person is related.