https://m.youtube.com/watch?v=zEGafGrMrdw

Look under the dash and seats at the metal frames if there’s rust there’s a good chance the interior has seen excessive moisture 

Look for unusual interior damage for the age of the car, and things that just seem weird or out of place. Pay attention to the smell of the interior. If it smells musty, it's probably been flooded out. My step dad unknowingly bought a flooded out 2013 Ford Escape from a dealer in 2015 and the car was "Certified Pre-Owned". The car only had 20k miles on it. He noticed the back passenger door frame was slightly dented at the bottom and the paint scraped, there were a ton of air fresheners hanging from the rear view mirror, and the power seats didn't work. 

The Escape drove OK initially, but about a month later, the tranny blew up and shifted violently into 2nd gear Limp mode. He towed it back. There was mud and water in the transmission. They tried to put a new transmission in and give it back, he forced them to sell him the loaner Escape they had given him while they checked out the car. They fought him tooth and nail for that, but their "Certified Pre-Owned" guarantee, and the financing bank refusing to pay for the flooded vehicle forced their hand. 

We figured out that the back door damage matched the foot of the backseat. It was from someone roughly pulling the seat out to clean up water and they damaged the door. The Carfax was clean. It listed the prior owner as a rental agency and it was a Chicago, Illinois car. Unknowing to the dealer, the car was in a storm that flooded a ton of cars.

https://www.youtube.com/watch?v=ljXqZ4Do6uQ

Check for signs of rust under the dash, along trunk welds, etc. Also pay attention to any mildew scent or any strange scent that seems out of place. Scotty has a few videos on this.

A few tell tale signs are finding rust/corrosion in places where there should be none. Places like the seat bolts & rails, the cabin fuse box, the seat belt buckle which have fresh orange rust are major red flags.

Another place to check is the metal portion of the chassis just behind the boot latch (just above the rear bumper), which is usually covered with rubber beading. Once rust grows there, it’ll continue to deteriorate and you’ll find rust particles on the inside of the rubber beading. 

If you spot flakes of mud in places like the speaker grills, tweeter grills, deep inside the seat belt buckle, in the HVAC vents (check with a flashlight), it usually means the cabin was fully flooded.

As others have mentioned, spotting an air freshener inside the vehicle should raise the first red flag. 

Check for fresh rust (orange in colour & will stick to your finger) on bolts, bare metal parts around the vehicle. 

I can go on, but spotting one or more of the above should set alarm bells ringing..

rust forms in hidden areas, especially at the edges of sheet metal where it isn't protected.

Pull away things like rubber gaskets and grommets, weather seals etc. (like around your door). Lift the cover in the trunk where the mini spare often goes. Moisture like to collect in there too.