General Motors transmissions can be identified by casting numbers, manufacturing codes, and the shape and number of bolts on the transmission pan. Casting numbers are most commonly used to identify manual transmissions, especially in older GM vehicles, while the transmission pan is most often used to identify automatic GM transmissions.
Casting numbers identify the specific manual transmission, including drive ratio, number of gears and compatible components. GM vehicles manufactured after the mid-1980s have manufacturing codes on a sticker inside the glovebox or the driver's side door jamb. Codes beginning with "M" relate to the transmission. MonsterTransmission.com features a guide to help identify visual cues for automatic transmissions.