Most motorcycle tires have wear marks molded into the treads. Quite often (but not always), the whereabouts of the wear marks is pointed to by little triangular “arrow-heads” on the sidewall.
The wear marks themselves are little bumps in the rubber at the bottom of the tread grooves. You won’t be able to see them on new tires, but as the tire tread wears away, you’ll see these little raised bumps at the base of the tread groove at a few places around the circumference of the tire. Once the tread is worn down level with the wear markers, then it’s *definitely* time to replace the tire (if not before).
Alternatively, if you have a tread depth guage, ignore the wear markers and replace the tires when there’s between 1 and 2 m.m. of tread left.