How to see if project boat is solid?

[mwe-maxxowner]

I have discovered my boat needs transom, stringers, foam, and floor redone. I found a boat sans motor and trailer similar in size for 500 on craigslist. Looks better, I like the layout, and the seating better than mine. How do I check for everything being good to go? I know how to test the transom, but what about the stringers and foam? Obviously I can stomp around the floor, but also what if he (understandably) doesn't want me drilling into his transom? He says the transom and stringers are solid fiberglass, so I don't think he knows much about it.