Flood damage often results in mold, because there are parts of the car that are simply inaccessible and can’t be cleaned. The car may seem dry but there are bits of still water in the small cracks that will build up some mold eventually. So get a good whiff around to detect any hints of it. Also, if the seller deodorized the car, they could be trying to cover it up to pay close attention to that trick.
The water stains
A car that has been submerged during a flood will have a water line on the interior, indicating just how deep the submersion was. The carpets in particular (if they are still there) will be badly stained. Even if the car has been through deep cleaning, pop the hood and the trunk open and you should be able to see the water line under there.
New upholstery in an old car
Upholstery can be easily damaged by water so it is likely to be replaced by the person who flipped the car. If it looks too new compared to the car, let it go. You could be scammed.
Take a close look at the headlights and all other lights (break, blinkers, etc.). If you notice fogging or little pools of water inside, it could be indicative of water damage.
Inspect the car’s undercarriage. If it has any signs of rust or flaking, it has probably suffered flood damage. Old cars may have a naturally rusty bottom, but if the vehicle is relatively new and already rusting, it was probably damaged in a flood.