How to check if you need new tires

Tires get old and worn down. And because a tire failure while you're driving can be catastrophic, causing your car to go out of control or leaving you stranded in the middle of nowhere, you want to know when your tires are in bad shape so you can get new ones before something goes wrong.

Tread Depth: The tread on your tires should never fall below 1.6 millimeters in depth. If you regularly drive on slick, wet surfaces, you'd be even better off with twice that much. 



The Tread Wear Indicator Bar: Newer tires have tread wear indicator bars built into the tires themselves. These bars, invisible or barely visible when the tires are new, gradually begin to appear as the tread wears down. They appear as flat rubber bars running perpendicular to the direction of the tread itself. If more than one or two of these are visible on a tire, the tread is getting low.

Cracks in the Sidewall: Look for tracks or cuts in the sidewall.This could be a sign that your tire is developing a leak (or worse, that it's nearly ready to blow out). This is definitely something you want to avoid.

Bulges and Blisters on the Tire: Sometimes the outer surface of the tire begins to weaken. The result can be a bulge or blister that extends outward from the rest of the surface. This weak spot can cause a sudden blow out, and if you don't put the car in the service center before this happens, it may end up putting you in the hospital when the tire blows out on the freeway. 

Too Much Vibration: A certain amount of vibration is inevitable when driving, especially on poorly paved roads, but if you've been driving for a while, you probably know how much vibration feels right and how much means that something's going wrong. There can be any of a number of causes for the vibration -- maybe your tires are misaligned or unbalanced, or your shock absorbers are starting to go. But it could also indicate that there's some sort of internal problem in the tire itself. Even if the tire isn't the root cause of the vibration, the vibration could damage the tire and pretty soon you'll have a problem. So if your car has a bad case of the shimmy-shimmy shakes, especially if you notice this when you aren't driving on bad roads, take it to the mechanic right away to have it checked out.

Source:
http://auto.howstuffworks.com/5-warning-signs-you-need-new-tires.htm