Drive it and/or have it inspected by a dealer, include a compression test. The nice thing about a dealer inspection is it will give you an idea of how the boat operates and what, if any, costs will be to repair anything.
I work at a dealer and the inspection can include anything you want but typically we do
-trailer light inspection
-trailer brake inspection
-dash electronics inspection
-check for signs of overheat (exterior damage to exhaust manifolds, muffles, exhast flanges)
-check for signs of freeze damge (cracks or bulges in block and manifolds)
-steering cable inspection
-tether cord inspection
-make sure engine builds temp
-run in gear
-check drive shaft and strut
-check bilge pump(s)
-check ballast pumps
You can do most of this yourself too, I would recommend taking a list with you just to make sure you don't miss anything.