Go see Mikey or Tim at Gilbert Lake Marine in Caldwell. They will help you find a good used boat.
-Check bilge for excessive oil residue or water retention
-Upholstery stitching and appearnace of all seats
-If wood stringers, thoroughly check floor and bilge compartments for soft spots or dry rot.
-Electronics: nav lights, bilge pump, blower, stereo, horn.
-Engine compartment: Hoses in good condition, belts not cracked or worn, coils and plugs, check oil filter to see when last oil change was done, usually indicated by mechanic writing hours on the filter, check bilge for any leaks or excessive residue.
-Test drive everything! If you're looking for a direct drive, v-drive, listen and feel for any prop vibration, check turning in both directions, couple of flat out runs to keep an eye on temp guage to monitor operating temps and impellar operation.
-Lastly, look at the trailer! Check to see bearing buddies are okay and greased, bunk carpeting in tact, fenders, tires, straps, prop guard, a trailer will give you a good indication of how well the boat has been maintained, as well as how many road trips she has had.
Any qualified dealer can usually do an inspection for minimal cost and at least you will have the peace of mind of knowing that the boat is going to be worth your time and money...