How do I know if I have the correct TDC on the compression stroke?

 

Pull the spark plug out of the other cylinder, then turn the engine until it comes up against compression. With the plug out there is no compression on the other cylinder.

To do the second cylinder put the plug back in, pull the plug from the first cylinder you did, the turn until it comes up against compression - same deal, with the plug out there is no compression with that first cylinder.