Depends on how much error you can allow. The easy way is to turn the engine over slowly until you feel compression, then pull the spark plug and put a dowel or long screw drive in the hole till it hits the top of the piston. Then slowly continue to turn the engine until the rod stops moving up. You will probably feel the crank go over center just before the rod starts moving down. Just back it up a tiny bit and you are there. You might have to pull the starter rope housing off so you can get to the flywheel incase you need to move it backwards. Just be sure before you pull the plug that you are on a compression stroke or you can see the valves through the plug hole, then you can be sure both are closed.
Edit: You cant use a rod on an old Tecumseh flat head because the piston is offset but you can see the valves and if you look to the side you can see the top of the piston.
