Corrosion is a common affliction in flood-damaged cars. The damage you see today isn't the only ramification of rust: Corrosion continues to eat away at materials long after the car is dry.
Rust and corrosion are often visible. Look for signs of corrosion on metals both inside and out. If you see rust on screws, door hinges, hood springs, trunk latches or brackets under the dashboard, for example, you know those metals had significant contact with water.
To check even more thoroughly, open the doors and look at where the door meets the body. Corrosion often occurs in that corner. In fact, check all four doors, including the bottoms, inside and out.
Finally, use a mirror and look below the seats to see if the springs are rusty. Use the mirror again to check the undercarriage of the car for flakes, metal that's been eaten away and other signs of corrosion.