I'm curious, have you cycled your tank already?
Anyways, watch the fish for abnormal behaviour, look for white spots or streaks on them, watch for them rubbing against the walls or decorations frequently, see if they are off their feed, and such. If you don't see anything, chances are that they are fine. Don't treat your fish for something, unless you are pretty sure they have it.
When I feed my betta in the community tank, I always get his attention with his food, and lead him over to a corner away from the other fish, then feed him first. While he's occupied, feed your other fish. Chances are after he's done, he'll try to eat the other fishs' food, use your fingers, or a small net to keep him away. Also, fast him once a week, just incase he is over eating. :)