Insert a quarter into a tread groove with the top of Washington's head facing down. If the top of his head is not visible, your tires have at least 4/32" of tread and are fine for continued use. If you can see above the top of Washington's head, it is time to start shopping for new tires. Take measurements in three locations across the tire's tread: (1)outer edge, (2) center, and (3) inside edge.
The penny test is done in the same way, except that if you can see above the top of Lincoln's head your tires have less than 2/32" of tread, which is below the legal minimum and cause for immediate replacement. Tires worn to this level will also have visible wear indicators (thin bald strips) running from side to side across two or more tread segments.