Tires are an important factor in your car's safety and contribute to less effective steering, accelerating and braking as they approach the end of their life. Damaged or worn tires can increase the risk of an accident or breakdown that could leave you stranded. Tires that show damage or wear should be replaced immediately. Fortunately, tires are one of the simplest parts of your car to inspect.
As tires wear, the tread depth decreases. You can check the tire manufacturer's specifications for minimum tread depth, and a tire tread depth gauge is only a couple dollars.
You can also place a penny in the tire tread with Abraham Lincoln's head upside down. If you can see the top of Abe's head (his hairline), it's time to replace your tires. This is sometimes called the 2/32-inch rule. If your tire has tread wear indicators, check the little bar between the tread and see if it's flush with the tread; if it is, then it's time for a tire replacement.
Inspect the contact surface of the tire, determining if the wear pattern is even. Tires that show uneven wear may help diagnose a maintenance or mechanical problem.
Over- and underinflation can damage the sidewall in addition to abrasions while driving (contact with curbs, etc.).
You only need your eyeballs and a little bit of knowledge, although a tread depth gauge is useful and inexpensive, and a penny is also handy and less expensive.