Sadly most of us have timing chain driven set ups with our m42s so I am of not help with your m40.
Marks for TDC are usually on the external casing and look like 0|T
There are some mechanic generic methods of inserting a probe into the # 1 cylinder and notice it's highest point after a two 360degree rotations to determine TDC.
You might want to get a Haynes manual or a guide for you m40.
If your belt snapped while driving, and you have an interference engine like the other bimmers I know of, your issue is far worse than locating TDC
Good Luck