Use a gear a with known number of teeth N that's close to the gears to be tested as a standard. Draw a scale with a mark on each tooth of the standard starting below N and ending above N. Draw a bigger mark on N. Draw an arrow in the direction of descending numbers. Get the gear you are testing to mesh with the standard, align the mark in N with the line of centers. Draw a mark in the tested gear in the point of contact. Turn the gears so the standard turns in the direction indicated by the arrow you drew. When the tested gear has done exactly 1 turn and the mark is exactly in the line of centers again, the mark will serve as an index to the scale in the standard and indicate the number of teeth.