Apart from the usual 'condition' checks that you'd do on any bike, the main thing on the pre '97 1100's is that the gearbox doesn't feel like it's skipping (for want of a better word) in second or third gear under hard acceleration. Unfortunately the only way to check for this is a test ride, which will depend on the seller allowing you. Don't listen to the fear mongers who will crop up and tell you that your bike is a time bomb due its gearbox, most of them don't have actual experience of a pre '97 bike and are just repeating Internet heresay. I have a '94 1100, so I'm allowed to say this
If the gearbox does have 'the problem', it's not the end of the world - it can be refurbished by a specialist such as Steptoe or Scriminger.
When I was looking for my 1100, I wrote a checklist to try and make sure I didn't get seduced by a bike and miss something. With hindsight, it's well over the top but I've copied it below for a laugh
Brake discs (min 4.5mm)
Wiring under ignition lock (cracks / chafing)
Fork sliders (leaks / rust)
Wheel brace / fork leg pitting
Align wheels
Front shock?
Brake leaks?
Brakes free?
Oil cooler ok?
Front tyre
Check paralever play (12/6 9/3)
Paralever gaiters (leaks)
Rear brake disc?
Rear frame to gearbox mountings (cracks)
Centrestand to gearbox mounting (cracks)
Rear shock?
Oil leaks?
Exhaust condition
Back tyre
Tank paintwork
Side panels
Panniers
Seat condition
Seats remove?
Toolkit?
Tax disc?
Gear brake levers straight?
Oil level / condition
Cylinder head leaks
Rocker covers
Running
Starts cleanly
Oil pressure light straight out?
Charge light straight out?
RID works?
Noises (top end / rumble)
Electrics all work?
Instruments work?
Test ride
Pulls cleanly
Gearbox skipping 2nd / 3rd
