How do you know when to change car tires

Simple answer. Tread deph no less than 3/32 of an inch. (all treads). The standard test is a quarter inserted in the tread at the head. If the head isn't obscured, then you are OK.

Uniformity should be checked. Low dept in middle means high inflation. Low at edges means low inflation.

Then alignment needs to be looked at and tire wear patters are indicative of this.

Your climate and the severity of winter may influence when you replace.

So replacing tires is generally based on tread wear, but the entire suspension and alignment needs to be addressed at the same time.

You should constantly be paying attention to wear patterns, abnormally pulling to one side, shimmying etc, to maximize tire life. At the very minimum, check wear patterns and pressures when the seasons change temperature wise and when you plan to take a relaively long trip.

Keepitsimplestupid is right about minimum tread depth. However, that's when you absolutely have to change tires. If you're planning on keeping the vehicle, it's always best to change them earlier than legal minimum. You'll get better performance in rain and snow and better handling overall when you have deeper tread on the road. If you're going to replace them sometime in the next 60K miles, why not do it early and enjoy the benefits?