(7) (a) A person may not operate a vehicle if one or more of the tires in use on the vehicle:
(i)...
(ii) has a tread depth less than 2/32 inch measured in any two adjacent tread grooves at three equally spaced intervals around the circumference of the tire.
easy way to measure that depth is top of a penny to lincolns head is 2/32's. Most tires also have wear bars across the tread when the tread is level to those the tire is too worn out.
I might be giving more advice than you want, but legal and safe on tires is not the same. I feel that long before tread is low enough to fail inspection it is too low for safe travel on wet or snowy roads.