If My memory serves me right you'll want to check for.
1) Check the plug that is in it to see how it looks to be running, rich or lean.
2) Remove the stator cover and try and move the fly wheel up and down, looking for play which is a sing of loose crank bearings. Ask if the bottom end has ever been apart.
3) Drain a small amount of the oil and see what condition it's in, should give you a good idea of the maintiance the guys done or not done.
4) Try and make sure the frame is straight and not bent or too bent, because most a at least a little tweaked. Someone can chime in with A-arm to peg mount measurements of a straight frame?
5) Look the frame over for cracks, in the seat support, in the frame rails where they meat the down tubes and across the lower shock bridge, which you can see from under neath. The 88-90 had a change to the structure so they are not as bad for cracking as the 87s in that spot
6) check for a loose or broke steering steam
7) check for a
**** swing arm, especially the stock one where the chain slider upper bolt hole is
check are arms are straight and the bearings are in good working order
9) check for rust and damage to the frame tubing
10) We all know every bike is a trailer queen that the wife rode only every other the holiday weekend so it must have 10 hours max, right? That is until you get it and the piston comes apart 10 hours later, so check with the guy to see how many hours are on the motor and if the piston is OEM or has been replaced and when the last time it was replaced. Forged stock pistons will shatter a lot easier than a good wiseco so
you might be in it for a new piston, wrist pin bearing, alum thrust washers and some cometic gaskets to make sure it won't grenade on you.
11) A stock bottom end on a stock motor should get 100-200 hours on it before the bearings come loose and need to be replaced. If you plan on building this motor that will greatly shorten the life of the piston and the bottom end so if the motor is "all original" and you want to build it that could mean you'll be doing a rebuild once you start modding things and making more HP
12) I'd ask the guy about the clutch if it's ever been changed or altered. Most stock clutches with average use will be fine to continue using with good springs as long as you spec the plates and they are within allowable tolerances.
Decent compression should be no less than 150 PSI I think, higher than that the better.
That should cover you other than the obvious cosmetic stuff, if it looks beat it probably was.
Anybody have anything to add to the list of things to look for when buying a zilla?