There are several methods of determining the need for new tires, including using a penny to make a quick estimate of a tire's treads, inspecting for problems and observing any unusual vibrations when driving. Some newer tires come equipped with a tread wear indicator bar that can alert the car owner when the time comes to replace them.
When performing the penny test on tire treads, HowStuffWorks suggests placing a standard penny face down into the tread. If the head remains visible, there is not enough tread and the tires should be replaced. While 1 1/16 of an inch is a standard minimum for tire tread, driving on wet and slick surfaces requires more tire tread for a smoother and safer drive.
Tracks, cuts, blisters and bruises on the sidewall of the tires or their tread are all an indication that the tires need to be replaced as soon as possible. Similarly, a sudden change in the level of vibration in the cabin of a car is an indicator of a problem with tires, alignment or the car's shock absorbers, and should be looked at in a timely manner, especially if a big trip is looming in the near future.