Generally, the brake pads of most current models have a metal sheet similar to the one shown in the photo below. It is generally called a wear alarm. When the brake pads are worn to the position of the alarm, the alarm and the brake disc are in contact with each other. A sharper noise occurs, which means that the brake pads should be replaced. More high-end models have electronic sensors. When the sensor is worn to a certain position, the sensor will be disconnected, and the dashboard will prompt the brake pad to be replaced.