[orionredwing]

I'm surprised no one mentioned this yet. If you look at the physical tire itself, you'll see a rectangular indentation connecting between the treads. This is the wear marker.

When the thickness of the surrounding tread reaches the same level as the wear marker, you should plan for a tire change.

If your car is lowered and there's some sort of camber, if you corner a lot, or if your wheels are misaligned, also inspect the inner and outer edges of the tire treads. If these have worn down and look 'bald', it's way past time to change the tire, even if the center treads haven't reached the wear marker yet.