You can sometimes check the brake pad thickness by looking through the spokes of the wheel. The outside pad will be pressed against a metal rotor, and you should be able to see at least 1/4 inch of the pad. If you see less than 1/4 inch of pad, it may be time to have your brake pads replaced. However, it is always best to have a professional mechanic check the brake pads as there may be uneven wear that is not visible without removing the wheel.