Innovative Products Extended TDC Indicator
There are a few procedures that require knowing when a given piston is at TDC. There are a couple of techniques for measuring it, but this tool from Innovative Products makes it super easy. It is a spring-loaded indicator rod that threads into the spark plug hole and rides on the piston as the engine is (MANUALLY!) rotated.
Here it is in use.
As a tool, it really is overkill - I've got machine tools made to lesser standards - but it saves a bunch of futzing around. There is no question when the piston is at TDC as the indicator makes it very obvious.