The tire size and other metrics are located on the sidewall of your tire. See the following question for detailed information on how to read these numbers.
When you look at your tire’s sidewall you’ll see a string of numbers and letters; for this example we’ll use P215/65R17 98T. The 1st number, 215 in this example, is the tires width. The 2nd number, 65, is the aspect ratio in a percentage (65%); a lower aspect ratio means a lower-profile tire with a shorter sidewall. The next number, 17, denotes the tire and wheel diameter. So a 17 here means this tire is made for a 17 inch wheel. Finally, the last number and letter, 98T, denotes a Speed Rating and Load Index. Learn more about this from this TIA Article.
A quick and simple way to tell is by taking a penny between your thumb and forefinger. Place Abe’s head into the groove where the tread on your tire is lowest. If any part of Abe’s head is covered by tire tread, your tires are still safe. If you’re getting concerned though, it’s best to get them replaced, as your safety on the road will increase exponentially. When the inflation pressure is regularly maintained, and the tires are rotated every 5,000 to 7,000 miles, all four tires will typically be worn evenly and replaced at the end of their service life. Drivers who fail to maintain and rotate their tires often find themselves needing to replace tires on one axle before the tires on the other axle are completely worn. Click on the following linkes to learn more about Tire Maintenance and Tire Safety Inspection