After so many long, straight miles (*cough* Kansas *cough*) the tread on my rear tire had started to really wear down right in the center. It was pretty visible that it had been wearing down.
The way they showed me at the dealership to know if you need to replace your tire is of you look in the tread itself. There are little “bars” or strips that go across in some places. When you’ve run down to these, then that’s how you know to replace the tire.