Brake linings should be inspected periodically. A good time would be when you have your tires rotated. Most manufacturers recommend rotating your tires every 5,000 to 8,000 miles. There should not be an additional charge if the brake inspection is done when the tires are rotated. Check the lining material on all the pads, inner and outer and front and rear, of each pad. If the thickness is down to the thickness of a nickel or less at any point, they should be replaced. The caliper slides should be cleaned, also. Any springs or clips that are worn or missing should be replaced. Rotors should be checked for excessive wear or deep grooves and be resurfaced or replaced as needed.
Some brake pads have squealers to let you know when it is time for new brakes. On these models, you may hear a high pitch squeal when you are driving with your foot off the brake pedal. The noise may go away when pressing the brake pedal. This indicates you should go in soon for a brake inspection. If you feel or hear a grinding noise when braking (like driving over gravel), you need to try not to use the brakes until you get them inspected. You may be metal to metal and that gets expensive to repair and unsafe to drive.