How primitive do you want to get? Number one is usually the cylinder closest to the front of the tractor. For a gasoline engine you can pull the spark plug, put a dowel into the hole, and rotate the crankshaft with a wrench until the maximum amount of dowel is exposed. That will be TDC. It can be either the compression or the exhaust stroke but is TDC either way. If the rotor in your distributor is pointing at one at this point you are on the compression stroke. If the rotor is 180 degrees away this was your exhaust stroke.