Swimming. - Is the fish easy to catch, or does the salesperson break up a bit of a sweat?
When you place your hands near the top of the tank, do the fish come up and expect food?
Any "mould" like substance appearing on any of the fish in the tank?
Any dead fish in the tank?
Fish gasping for air?
Erratic behaviour for that type of fish?
One fish slower than the others?
Clear eyes, milky eyes, missing eyes?
Marks or scratches.
A big list.
Ask the salesperson how long the store has had the fish in question. Fresh arrival, or been at the store for longer?
Also note, that a lot of fish are highly likely to get certain diseases - Clown Loaches & Ick for example, and while they won't show signs at your LFS, when you take them home, they may get the infection. Not a big deal, easy to handle.