What is the earliest and best way to recognize that a trailer has started to skid?

AAMVA CDL Manual, Section 6.1.5 "Prevent Trailer Skids," p. 6-2

Once a trailer swings out of its lane, the manual notes it becomes very difficult to prevent a jackknife. Making a mirror check an automatic reflex after every hard brake application is the difference between catching a skid early and losing the trailer entirely.