The best way to notice if a trailer has begun to skid is to:

Question

The easiest way to become aware that your trailer has begun to skid is to observe it in your mirrors. Checking your trailer in your mirrors will also allow you to spot any skidding at the earliest possible moment.

Accepted Answer

Use your mirrors to determine if the trailer has left the lane.

Answer

Ask another driver.

Answer

Look at the skid sensor.

Answer

Feel the tractor move.