One way to tell when it is time for new tires is to check the tread wear indicators, which will appear when your tires have only 1/16 inch (1.6 mm) or less of tread remaining. These indicators are flat spots between the treads.
Tires should be checked for abnormal wear patterns periodically. Just rub your hand around the tire and feel for roughness, flat spots, or feathering.
Does you car need a Front End Alignment?
Check tire tread depth with this little gauge that can be purchased at any
local parts store.
You should Rotate Tires every 6,000 miles, or as specified by your vehicle manufacturer.