Assess the tread depth of your tires by placing a penny with Lincoln's head upside down in the tread of your tire. If Lincoln's head is buried between the tread blocks then the tire still has 2/32 of an inch of tread remaining and the tire passes the tread test.
If Lincoln's head is still visible, the tire is worn down beyond 2/32 of an inch and needs to be replaced. Most state laws require a tire to have a minimum tread depth of 2/32.
Perform a similar test using a quarter instead of a penny. Place the quarter between the tread blocks of the tire with Washington's head upside down. If the tire tread obscures Washington's head, the tire has at least 4/32 of an inch remaining.