On the ring, all the teeth are equally spaced out, except two of them. There is a wider gap between these two. Looking at the balancer/ring from the front, find this gap, and then count 5 teeth counter clockwise. If you can, do a little light sanding to see if just maybe the numbers are still there, or at least the dashes. If you can find the numbers, TDC is Zero. It will be right above the fifth tooth counterclockwise from the gap.
Here's a link that explains it with pictures..