I am trying to buy a flood damaged coupe. But it has drug out for months now. Losing interest in it. But I was going to do a complete restoration of the car as a keeper. I think an easy way to tell if one has been under water is look under the dash. Main cross member, all of them have some slight rust on them, but would guess a flood car would show real rust in that area. It can be seen by opening the driver side door and looking up into the dash along side the steering column, right side. I think there are photos in my Mallett restoration.
But if you are not willing to do everything to the car, I would avoid it. I know several that have flood damaged cars. Almost bought a Carbon Flash Redline. It was a nice car, had water over the front end of the car, at the bottom of the rotors, and was totaled. Some do not have much damage, but you have to look at them closely. The jest of this story is buyer beware.