Here is the tool, a dial indicator and magnetic base. About $50 for acceptable quality. (all web photos)
Here it is in acton on a car engine. Wiggle the rocker arm up and down and the needle will indicate the clearance.
Here it is checking a crank. Support the bearings on blocks (I used magnets to hold them steady) and spin the crank. The dial will tell the truth. My well used 66cc engine was within 0.007" at the worst spot (crank cheeks, side to side). Freehand beating it with a brass hammer got it within 0.005" and less than 0.001" on the journals. It was less than 0.002" there before I started.
Here is someone doing a HT crank in a lathe rather than on blocks. Either works, but I have a couple 2"x2"x4" steel blocks and 4 magnets.
News flash, I've just bought another camera. I'll keep it away from salt water and whales this time. Unfortunately the cases are already stuffed with epoxy and the crank installed and closed up. I put the seals in backwards to further reduce crankcase volume and drilled a breathe hole to the back of the crank bearings to keep the oil flowing. Would have been nice to have pictures for you. Sorry.
Steve