Bottom operator calls a last chair/gondola to the top. Some ski areas require a cone or other marker to be placed on this chair. After last chair, operator watches lift to make sure no one loads. When last chair reaches the top (or back to the bottom on a lift with downloading,) lift is stopped. Some ski areas then have a person ski down the line to verify no one is on the lift.
On gondolas with cabin parking, it is somewhat moot because anyone left on the lift would go in a nice warm warehouse anyways!
A somewhat similar process is used at many Disney attractions, it's common for 1-2 the cast members at each attraction to "ride down" the attraction, boarding just after the last day guests so they know the attraction is clear and check for any show issues.