First, check for wear by looking at your brake pads through the spaces between the wheel's spokes. The outside pad will be pressed against a metal rotor.
Generally, there should be at least 1/4 inch of pad. If you see less than 1/4 inch of pad, you may want to have your brake pads inspected or replaced. If you have more questions regarding your brake system check out our video
Understanding your braking system.