Originally Posted by polarbill
The floor will be soft. To check the stringers take a socket set with you. Just try and tighten the motor mount bolts. If they spin without tightening the stringers are mush. If they bite down they should be ok.
Unfortunately, this only tells you about a small portion of the stringer. Pull up any section of flooring you can, tap on it with a hammer and listen if it feels solid in addition to turning the lag bolts.
Thats about all you can do thats non-invasive in a boat you dont own yet.