You need to perform a valve clearance check on a cool , warm to the touch engine, an engine that has not just been run .
It would be good idea to check the torque of the cylinder head nuts, before you start the clearance check seeing as you are not the original owner and who knows what was done to the bike before you got it .
So you are going to need a torque wrench .
Here's a youtube video on how to adjust valves on an R100 .
Warning, the language is ' colorful ' at times !!!!!!!!!!!!!
https://www.youtube.com/watch?v=VExx_2kIc9cI have an '81 R65, original owner, the '81 model year airheads were the worst for valve recession .
I had the heads rebuilt at 45,000 miles, around 73,000 km .
The exhaust valves were close to failing, they were worn razor thin at the contact area with the valve seat .