In "The Love Song of J. Alfred Prufrock", at what point in the poem do you realize that Prufrock will not declare his love?

The entire poem seems to reveal his hesitation, his fear, his insecurities, and the excuses that he gives himself for NOT asking his question.  He vacillates back and forth, but most of the time expresses his trepidation at asking.  His hesitancy and fear are a palpable presence in the poem.  He questions himself over and over, "do I dare?"  He gives all of the reasons he shouldn't, talking about the trivial nature of the conversations at these parties, about how they will reject him, about how they will laugh at and mock him, how they will over-analyze him and "pin" him under their scrutiny.  The entire poem is an expression of doubt.  So, it is easy to guess or hypothesize that he's probably going to chicken out.

That feeling becomes a bit more concrete when he says his head is brought in upon a platter and that he has felt death (the eternal footman) waiting for him.  When he starts asking "and would it have been worth it after all" to ask the question, here his decision materializes more solidly.  He is weighing the cons of asking, worrying if it will be worth it.  At this point, we can probably guess.  His excuses are more solid and tangible.

The actual declaration that he will not ask her the question though comes around line 125-ish, when, asking several times if it would have been worth it, he declares with emphasis and finality, "No!"  He states the answer right there.  He has decided it is not worth asking his question.  After all, he is no "Hamlet" who is profound, princely, and inspiring.  He is "a bit obtuse", a "fool", and ridiculous.  He decides at this point to not ask, to let it go, to feel sorry for himself, and to write long poems about his longing.  :)  I hope that helps!