Depends on the system, to be honest. There's so many different systems out there trying to get around the original Bosch patent that there's no way to tell what is going to be on what car. The majority of systems look at system hydraulic pressure, and also at individual wheel speeds. Some look only at front wheel speeds. Wheel sensors are usually inductive (teeth on the hub or disk brake), hydraulic sensors are usually piezo or contact in the brake lines. There's usually a servo-actuated relief valve to drop the brake line pressure, and the CPU doing the AB looks at the line pressure to check the line pressure has been relieved, then goes to have another look at the wheel speed of the wheel it thought was in trouble. The system can usually be checked through a diagnostic at the garage. This would eliminate sensors and the CPU: it would not eliminate other failures such as glazed pads, contaminated pads, damaged (undamped) shocks, extreme wheel bearing wear, extreme alignment problems, etc. etc. etc.

Golden rule - tires or brakes, DON'T wait, get them checked!!!!!