What are signs that a car was a flood car?

Depending on where is was, or how bad the flood was, the signs will change. The usual signs are an odor, usually pretty bad on the inside, especially from the upholstery. If the water had salt in it, you might see salt residue in the engine compartment and on the undercarriage. Also, there will possibly be rust where there isn't usually rust: inside door panels, underside of hood, radiator...

The main problem with flood cars is that the title might not be clean. The engine is water-tight (if it's not you really have some problems, and in that case, flood damage is the least of your worries), so the engine should be fine, but the rust in other places can be a big issue.