[Hozer 88GTConv]

You can shine a decent light down each side of the top well and see if the top seals of the cylinders are leaking. The nylon bushings that support each cylinder are easily seen too.
I would cycle the top several times, that is because if the guy has a leak, he may just top it off and hope you don't cycle it too many times.
A slow leak won't make itself known for sometime. Actuate each top latch checking for binding. Check the roller pins that hold the j-hook on the latch. This is a high failure rate point on the latch. If they fail, the j-hook loses its spring and won't seal well.
Check the top pads inside of the top and look for rubbing/chafing spots where the linkage may be coming into contact with the top.
Examine the weatherstriping, although it will most assuredley have some dry-rot issues, check the fitment at the joint of the quarter-glass and window. Big leak area. Check for fitment seal at the top window too. Any gap and you get wet.
Welcome to the club...