Welcome to the forum.
Drive the boat. If you can't test drive the boat, it's value should be half'd, simply because you can't tell what you have.
Walk around inside the boat feeling for soft spots under foot. If there are any, half the price, because the sole and likely the stringers are rotten. Turn on and operate every device on the boat. Check the fuses or breakers and look for shoddy wiring repairs (very common and indicative of the boat's care history). If you can splash the boat, insist on lower the drive into a garbage can full of water and fire the puppy up. It should fire right up, no smoke and it should engage in forward and reverse without any grinding, clanking, etc. Check for tilt/trim hose and line leaks. Hows the pump sound. The general condition of the boat should give you a clue about the owner and in turn the care the boat has seen. If it look terrible, it probably is.
Take someone along that knows boats, has owned several and is knowledgeable about drives to.