What is the best way to tell if your trailer has started to skid?

A see it in your mirrors.

explanation

The earliest and best way to recognize that the trailer has started to skid is by seeing it in your mirrors. Any time you apply the brakes hard, check the mirrors to make sure the trailer is staying where it should be. Once the trailer swings out of your lane, it’s very difficult to prevent a jackknife. [Commercial Driver License Manual]