ORIGINAL: Diablo-RCU
The most accurate way to find TDC is to use a piston stop (a bolt screwed into the spark plug hole that stops the piston about half way in the cylinder). Use a degree wheel and pointer. Turn the crank gently one way and then the other. Right in the middle is BDC. 180 degrees away on the degree wheel is TDC. The piston hardly moves at all near TDC, so that's why the piston stop is more accurate.
Tim
Once again the voice of time honored practices...
I agree with Diablo, this is the most accurate method available for locating TDC, bar none IMO. And you don't need any goodies or measuring devices.
If you need to do this most likely your checking and or setting the timing. Thus the need for a degree wheel. C&H Ignition has a down loadable degree wheel on their site together with instructions which apply to many engines and ignition systems.
The stop can be manufactured from an old sparkplug by removing the electrode and porcelain and tapping it for the appropriate bolt. In some cases you can simply find the correct thread type/size. Some are 14mm...