A long, properly shaped coat hanger or welding rod inserted into the cylinder through the plug opening will let you know when you are at TDC. Speed shops have, or can order, a TDC stop which screws into the plug hole much like a spark plug. The center is tapped and an adjustment bolt with a rounded tip is used to block the piston. When you back it out to where the piston will clear you then turn in intil it just barely stops the piston at TDC.

In years past I have made one by gutting a spark plug, tapping it and modifying a bolt to accomplish the same thing.

Tim