A good place to start when buying a used car is to talk to a reputable dealer. Long-time dealerships with good histories won't risk their reps by ripping off their customers. With any dealer, ask if the car you're eyeing has been damaged by floods, and get the answer in writing. Anything less than a firm "no" or any hesitation to commit to that answer in writing are causes for concern.
Ask the dealer to let you look at the title, too. You can see if the car came from a flood-damaged region, and you can check for a stamp that reads "Flood" or "Salvage," required by law in some states.
Finally, beware of used cars with price tags far below market value without explanation. There's usually a reason.