Flood cars aren't exactly hard to spot.
...then again, I've seen people obliviously buy cars with bad engines and transmissions.
But if you actually look, you'll see the signs. Unless someone went through the trouble of A) finding flood cars with clean titles and B) doing a shitton of cleaning and replacing parts. People who unknowingly buy flood cars are not being fooled by geniuses; they're just not bothering to examine the title properly and/or taking a rudimentary look at the car.
Pull back a corner of the carpet. Look under the dash. If the interior actually got flooded, especially to a major extent (above the seat bottoms), you'll see signs of it everywhere. And again, there's not a lot of profit in buying a flood car, stripping the interior, and cleaning/replacing everything.
Engines can be a little trickier, though. But if all that happened was that the engine sucked down water, it's not so much a flood car as a 'drove through a huge puddle and crammed lots of water into the airbox' car. And that can happen without a flood.