Depends on the State, but usually "no firearms allowed" signs on private businesses don't carry the weight of law. If you are concealed, they shouldn't know, but if they do find out you are carrying the most they can do is ask you to leave. If you stay, you can get a trespassing charge, and even if they do ask you to leave they may call the cops anyway, and you don't know if the cop will know the law, so it's really a weird gray area.