Try the Bluenose test: place a dime in the tire’s groove with the Bluenose’s Sails facing down . If you can see the top of the Bluenose mass and sails, then your tires have sailed long enough and needs replacing. However, this test will not work with performance or off-road tires.
Tires are manufactured with .wear bars. The bar becomes exposed when there is less than 1.6 mm of tread depth remaining on your tire. Tires must be replaced when the wear bar is visible and even with the neighbouring tread block.