Walking around on it will tell the tale...if you're looking at a cuddy with a full windshield, get up on the bow deck ahead of the windshield and walk around there as well including around the bow rail mounts My transom test is tilt the motor up and apply your weight across the lower unit...no noise means its OK...snap crackle pop and the owner's freaking out, walk away.
It's not just these boats...I considered a 19' Grady recently...30 year old boat...soft as it could be and with a fuel tank issue...a lot depends on how the boat spent those 30 years...tied up at a dock or covered on a trailer
Take your time selecting and don't fall in love with a particular boat...until AFTER you buy it...