Harry,
You could talk to the owner and get him to discuss how and how often he used the boat. For many users, 100 hours per year would be a lot. If the owner went out 4 hours per day for two days every weekend, that would be 400 hours. But, for many users, 100 hours per year would be a lot. Slow cruising is less stressful than full throttle pulling water skiers.
Also ask about when and where he had the maintenance done.
Unfortunately, just asking does not guarantee honest replys, but it may give you some indication. You can also get clues as to condition by the general maintenance of the boat and trailer.