All Waltham pocket watches have a serial number stamped on the movement (the "works") inside the watch. The serial number can be checked against an online Waltham pocket watch list, which will give the approximate year of manufacture. Waltham pocket watch serial numbers can be accessed by opening the case back. To open a case back determine whether the watch has a screw-down, hunting or clamshell case.

To unscrew a screw-down Waltham pocket watch case back, locate the seam between the case and case back. The case back is the rear screw-down lid of the watch. Place the watch face down in the palm of your hand. Grip the edges of the case back with your fingers and rotate the case back counterclockwise until it's unscrewed from the case. Use a rubberized jar-lid remover to help unscrew particularly stubborn case backs.

Press the crown down at the 12 o’clock position on a Waltham pocket watch featuring a hunting case. Pressing down the crown releases the spring-loaded latch that opens the front cover over the crystal and the case back cover. Carefully wedge a blunt blade or your fingernail between the case and back cover to help pry it open.

Place your fingernail or a blunt blade under the case back lip at the 3 o’clock position if Waltham pocket watch has a clamshell case. Use the fingers from your other hand to grasp the pendant at 12 o’clock, where the stem that connects to the movement is held. By pulling the pendant and lifting the lip, an inner ring together with the movement will separate from the case.

Examine the movement with a jeweler’s loupe or magnifying glass. On one side of the movement is the Waltham logo and trademark. A model name may be engraved below, such as “Riverside” or “American.” At the edge of the movement is a set of numbers up to eight digits. The fewer the digits, the older the watch. Serial numbers begin in 1852 with 50 and exceeded 35,000,000 in 1957, the last year of manufacture.