As stated above, if the boat works properly they will let you water test it! Another thing to pay attention to is the cooling system pee stream running out the back of the motor. If it runs strong you're likely okay, but in looking for my last boat I found several that spit and sputtered water instead of a good solid stream. This can mean anything from bad water pump impeller (couple hunderd with labor, $20-$80 if doing it in your driveway) to a cooling system full of sand (not good) Also check the trailer over, leaf springs, wheels and the trailer frame can easily rust enough to break and leave you in bad shape, if it (trailer) has bunks check the condition of the wood and carpet, if it's rollers check them over when it's off the trailer to water test. Look at the wiring in the boat, on the water isn't where you want things to short out, if it looks untouched and slid you're probably okay, if it looks like goober got into it with the butt connectors and wire clippers it's not what you want. Make sure all of the electonics work, including the nav lights, gauges, and bilge pump. IF the speedometer does not work, don't be suprised, I upgraded to a gs speedo on my big boat because the standard unit was useless, the one on the flats boat stays packed with salt/sand so it's trash too. But the tach and fuel gauges are very important. Also, if you go with an aluminum boat that is full welded but uncarpeted, check the rivets that hold the decking into the hull, this will give you an idea how hard it's been ridden. In the river I never popped a rivet out of my old boat, in Bouge Inlet it would always break 7-8 rivets. At least they just hold the floor and casting deck, had it been a riveted hull it would've fallen apart.
lol, thanks again