1. Feel the brake surface with your finger. If it feels noticeably concave, it's probably time for a new rim.
2. Check out the spoke tensions with your fingers. If certain spokes are noticeably tighter than the opposing ones, or if your rim seems to be out-of-true in the direction of looser spokes, you have a bent rim. If that's the case, your rim is living on borrowed time.
In either case you can either plan a rim replacement or wait until the rim demands to be replaced. Rim demands usually happen when you're a long way away from your car.