Here's Bill Gardiner explaining how to check your tire's tread depth using a quarter or a tread depth gauge.
With the quarter, place it in such a way that you can see the Cariboo and, with it's nose pointing down, does the tread cover the nose? You may need replacement tires.
With a gauge, find the most shallowest groove of the tread, insert the pin of the gauge until the base is flush with the tire.
If your tread depth is low, make sure to speak with a qualified technician. Easy as that!