If you do not keep a “repair/maintenance index card” on your locos and you change out some but not all the axle gears on one loco., put a spot of green paint on the new/replaced axle gear stems.
Even though this paint will not mean much later because when the cover comes off to check for a cracked axle gear, all axle gears should be checked anyway. It would just let me know if an older axle gear cracked or a replaced one, the one with the green paint spot.