When you change your tires, you can rely on the wear indicators located at the bottom of the tread grooves (their position is indicated by an arrow or a pattern on the sidewall of the tire). If the rubber is equal to the indicator, the tires must be changed.
You can also purchase a depth gauge (available in specialty stores and many mass merchandisers), which will allow you to determine whether your tires can withstand another season quickly.
If you don’t have this tool and your tires don’t have a wear indicator, you can always opt for the popular 25-cent technique. To do this, insert the coin into the bottom of the tire where it seems to be the most worn, with the caribou’s snout facing down. If the end of the tire remains fully visible, your tires are too worn.