Another way to tell when the piston is at the top is to use a wooden dowel about 10"-12" long that will fit into the spark plug hole and rest on the top of the piston. Rotate the engine by hand until the dowel rod comes all the way up and just as it starts to go back down you know that the piston is at the top. Basically the same as Hellomrwilson said using the screwdriver trick.
