The four signs
Most pads carry a small metal tab that squeals against the disc when the friction material is nearly gone. A pulsing pedal usually means a warped disc rather than the pads. A low, soft pedal points to fluid or air in the system and should be checked the same day.
Check the thickness
Look through the wheel spokes: if the pad material is thinner than about 3 mm, order a new set. Pads are sold per axle, so one set covers both front wheels.
Pads or discs?
Measure the disc with a caliper. Every disc on this site lists its minimum thickness. Below that number, replace discs and pads together; the bundle on the product page saves money and one delivery.
