With the engine assembled you can not do it 100 percent accurate....
pull all 8 spark plugs so the engine is easy to turn with a breaker bar. Rotate number one cylinder to tdc the best you can, take a small 1/8 rod and probe in the spark plug hole. The fun part is you are trying to rotate the crank till the piston is at its farthest upward point. Now make sure your timing mark on your balancer and your timing marker line up perfectly.
Hope this is understandable
P.S. dont just stick the rod in the hole and rotate the crank..... you dont want to smash it between the head and piston, you have to get the piston almost at the top before you go sticking it in there, carefully.