pull the door panel and roll the window up.
If it's a flood truck, there will be a grimy waterline inside the door. Nobody ever cleans in there, even though it's just a 15 minute job.
With that said, I know a guy who has been buying water damage cars for years. He has developed a knack for drying them out. He buys KNOWN flood damage cars with clean titles. He is always sure to locate the water line and will not buy if the ECM itself has been underwater (or the oil fill or the dash/climate control units). and then he cleans them up, removes the interior, unplugs all the connectors. Sets up table fans and dehumidifiers and dries them out in a small rented warehouse two at a time. Then he puts them together and drives them for at least 6 months to sort out the bugs.
Then he sells em.
He's gotten to where he specializes in Audi turbos and Dodge trucks. I guess because he has found that he gets the best deals on those for some reason.
He, his wife, his two daughters, and his son all drive flood damaged vehicles. then they send em on down the line. I asked him one day if he had trouble sleeping by selling flood cars to people and he said Nope. When I sell em they are well sorted out. He's not had any of them come back to my knowledge. This guy is an individual. Not a dealer.