At Fifth Avenue, the hawk watchers are asking what happens at every nest, when will the eyasses (young hawks on the nest) fledge (fly off the nest). To answer this question requires some basic science, some behavioral science and lots of luck.
The basic science is that we know that generaly hawks fledge after 42-46 days. For some reason this can be longer for city nests, say 45-50 days. We're not sure why we have this variation. It could be that we can count hatches more accurately in the city or that building nests without the opportunity for branching activity by the eyasses prolong the period. Hawks also need to be physically mature to fledge. So hawk watchers look at feather growth but more importanly tail length to judge if a hawk is ready to go.
On the behavioral side, we look for lots of jump-flapping and movement around the nest. We also look at the temperment of the hawks. Like college age childern, some look like they are eager to move on and others look like they won't move out at all.
But in general, seeing a flege is about good luck and putting in the time to be in the right place at the right time.