Gate agents have gotten better at weeding out many who don't have outward symptoms. Sure they cannot legally ask why, but I've heard several agents ask the passenger "OK what kind of special assistance does your condition require". If they say early boarding, an aisle seat, extra time to walk down the jetway, etc, I've seen the FA write Family Boarding (between A and B group) and refuse preboard passes.

If they say someone has to help them down, they cannot walk far/long without falling, etc, then that is, and always has been, an automatic preboard sleeve.

Air travel seems to at least temporarily cure many handycaps: it's not uncommon to see less than half the wheelchairs used to deplane than to board the aircraft.