Harmonic balancer

Post #16, verifying TDC with a degree wheel. My post, #15, was based on the statement made by the OP in post #1 that he was changing cams in a long block. Unless the meaning is different between our two countries, a long block engine has the heads installed.
So in this situation [ heads in place ], how does a degree wheel verify on it's own that the piston is at TDC, in order to verify the TDC mark??