Top Dead Center is when the piston is at the top of the stroke and the valves are closed.
If you already adjusted the valves, then you should be able to see them to make sure they are opening and closing properly. I think the sparkplug sparks twice during the 4 cycles. Just make sure the sparkplug sparks right before the piston is at the top of the stroke.
What engine? How old is the engine? Honda engines use a rubber timing belt.