The answer to your question is different at different levels, as has already been said.
At the high school level, the QB (if the HC trusts him to) is probably limited to about 3 pre-snap things:
1 - ID the MLB (to see if the defense is balanced)
If the defense is unbalanced and a run to the crowded side is called, he might do a simple audible. Say it's a run to the right and the safety creeps way up on that half of the field, he'd yell something like "check - Lucy" which flips the same play to the left. He'd say any word starting with L.
2 - Is the coverage man or zone (pre-determining if he may be throwing more to a spot or area vs leading his receiver(s) with tight coverage)
Again, for high school, a man in motion gives it away, but so could positioning. If a safety is walking up and aligning himself with a slot receiver, that man suggest man. If both safeties are ready to backpedal at the snap, they're probably in a cover 2 zone.
3 - generally looking for an obvious blitz - if he can tell where they're coming from, then he can automatically throw to the vacated space - a quick dump off to the TE if the LB who would be covering him or that zone threatens blitz.
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If a QB can do all that with some consistency in HS, he's good and if he can do any more than that, he's probably a name guy on recruiting rankings. I doubt many of the super-athletic guys can do all 3 consistency (because they don't have to, in order to be successful).