Rolex GMT-Master II 40 mm is the reference in this note. Inspect the crown and the case flank as a pair. The crown should sit centered between the guards, and the guards should be the same height. A guard that is shorter has been filed or knocked. The tube around the crown should not show a pale ring of bare metal unless the whole flank is brushed the same way. Run a fingernail along the lug top. Sharp edges that suddenly go round are a polish job. Polishing is common on a used case and it changes the shape, so describe it instead of ignoring it. Open the clasp and read any numbers stamped inside. They will not always match a serial you hoped to see. Write what is there. Look at the end links for a gap at the case and for screws that are chewed. A chewed screw means the bracelet was removed with the wrong tool. That is a useful fact about the life of the piece. It is not a reason to invent a story about the rest of the watch.
How it differs
Rolex GMT-Master II 40 mm has a 24-hour hand and a bezel marked for a second time zone. Check that the 24-hour hand agrees with the bezel when you set them together, and that the triangle or pip at 12 is on the dial axis. People change the bezel insert and the bracelet. A two-color insert should show the same wear on both colors. If one half looks newer, the insert was replaced.
What to check on a used piece
What to check on a used Rolex GMT-Master II 40 mm starts with the engraving. Measure the case. Listings often say 40 mm, and the ruler is the check. Then check end-link play, clasp fold, and whether the hands pass each other. There is no price in this note.
Common mods
Common changes on a used GMT are a replaced 24-hour insert, a swapped bracelet, and a 24-hour hand that is a different finish from the hour hand. Keep the reference engraving as the name of the watch, and list the newer parts beside it.
What people change
On the Rolex GMT-Master II 40 mm, people change the bracelet, the case, or the movement.