The Rolex Sky-Dweller carries an annual calendar and a second time zone. The month is read from the ring that sits off center on the dial. People who know the model look there first, before they look at the bracelet.
How it differs
A GMT-Master II uses a rotating 24-hour bezel. The Sky-Dweller does not. Its second zone is set with the ring on the dial, and the month moves only when the year needs it. The case is larger than a Datejust, and the lugs are long. A used example can look bigger on the wrist than the millimeter count suggests, because the bracelet and the end links add to the length.
The crystal is a box shape over a busy dial. Reflections hide the month ring if you only look straight down. Tilt the watch. The ring should be readable, and the 12 marker should still agree with the hand stack.
Common mods
Bracelet work is the usual change. End links that were made for another case will leave a bright gap at the lug. That gap is the thing to write down. Crystal replacement comes next, usually after a chip at the edge. The new crystal has to meet the bezel all the way around. A movement service is less visible: after it, the calendar should still change the date near midnight, and the month ring should not sit between two month names.
What to check on a used piece
On a used Rolex Sky-Dweller, check the bracelet end links where they meet the case, the crystal over the dial, and the movement. Set the local time and the off-center time once, then watch the date. If the date jumps and the month ring does not match the position you set, say that in the note. Do not invent a service history from a polished lug.
The notes for those parts are the bracelet, the case, and the movement.