This question highlights many of the issues and insecurities that Prufrock talks about within the poem. "So how should I presume" is Prufrock asking, "Who am I to ask such an important question, to feel like I am important enough to feel cared about by someone?" He feels completely insecure in himself; because of this insecurity, he backs down and is wishy-washy in asking speaking his heart to the woman he is thinking of. He doesn't presume to be anyone of any importance, and only people who are confident in themselves and their place in the world are comfortable asking life-changing questions such as the one he is wanting to ask. This question is not the only place he ponders his insecurities:
"Time to turn back and descend the stair,
With a bald spot in the middle of my hair—
[They will say: "How his hair is growing thin!"]
My morning coat, my collar mounting firmly to the
My necktie rich and modest, but asserted by a
[They will say: "But how his arms and legs are
This passage empahsizes his lack of confidence in his appearance and the fact that he feels people will mock him and ridicule his presumption that he is worth considering.
He also feels that he
"should have been a pair of ragged claws
Scuttling across the floors of silent seas,"
comparing himself to a crab at the bottom of the ocean, an unflattering comparison to say the least. He proclaims later that he is not prince Hamlet, "nor was meant to be." He is not eloquent, interesting, profound or impactful. Instead, he is "obtuse...ridiculous...the Fool."
So, "how should I presume?" reflects his attitude towards himself, that he feels he is insignificant, just a tiny crab of a human being, not important enough to presume that he will be considered kindly, or with any measure of significance by the person he is afraid to speak to.