Bill, not sure just what your asking... but for starters carry and air gauge and multimeter. Begin with checking the battery, engine off... 12-12.5v, engine running... 13-14.5v
While there you are looking to see if wiring has had major add ons, splices, etc. Then check tire pressures... it gives a great big hint how the current owner maintains it. If you find low tire pressures, sure sign he does not stay on top of things.
Of course the obvious fluid level checks.
I'd start there and maybe pull the caps on clutch/break fluids. observe color and levels. If you really want to get idea of engine condition and current owner does not object.. then use compression gauges and check that out.
If your asking about taking to local shop for inspection, then your on your own as very few will work on old wings.
Hmmm, guess I'll go read it too, LOL I never notice that buyers guide.
I also bought mine without knowing much about it. I knew it had a few problems, but I have been working on bikes all my life, and the big things, like the engine and frame seemed fine, and I got it really cheap.
If I had known what I know now, I would have bought a carbed model in slightly better condition. Hopefully this one has a lot of miles left in it, but it has been nothing but one problem after another since I got it. I will almost certainly get yelled at about this, but I would definitely avoid fuel injection. It is basically a pretty solid system, or at least the design is good, but it contains some VERY failure prone parts (failure prone after 25 years) that aren't made anymore. I'll bet a lot of those with fuel injected bikes have the same problem I do, and just don't know it yet.
Welcome to the forum. I didn't know much about my 1800 when I got it, but since I joined this forum I've learned quite a bit. Echoing what is said above, I would check tires (wear, pressure, cracks, etc.), fluid levels/color, hose conditions, lights, and anything else that might be past its lifetime. I would ask the PO about when the last time the oil and filter were changed as well as the final drive oil, and if the shaft has been greased with moly 60. I would also look underneath as best I could to make sure that nothing there is leaking, broken, etc. Just a few ideas.