Related: How Do I Know When to Change My Brake Pads and Rotors?
Sometimes, though, you may hear squeaks or squeals because the brakes are coated with moisture, light surface rust, dirt or grime, particularly after the car has been sitting either overnight or for several hours. Those noises might go away after you apply the brake pedal a few times or more (and the brake pads have cleaned the offending substance off the rotors or drums). Noise can also be caused by a loose brake pad or caliper.
If you hear a high-pitched scraping sound, that could be pad-wear indicators sounding the alarm that you're late in getting your car's brakes fixed.
Because there are several possibilities, the best way to tell if you need brake work is to ask a repair shop to inspect the condition of the entire brake system, including the rotors (or rear drums on some vehicles), the brake pads (the friction material that squeezes against rotors or drums), the calipers and other hardware, and the master brake cylinder and fluid lines going to each wheel.
Your ears and how the brakes feel and perform can tell you a lot about the condition of your braking system, but a repair shop can tell you more about what's wrong and what needs to be fixed. Excessive brake pedal travel could mean worn pads, for example, but it could also be caused by low brake fluid. While the symptoms may be the same, the treatments are very different.
A repair shop should not only eyeball the brakes to see what's wrong, but it should also measure the thickness of the pads and rotors and whether they're evenly worn. Manufacturers have different recommendations for when brake pads should be replaced, but as a guideline, some shops recommend new ones when only 20 percent of the original thickness remains. Others say it's necessary when the pad is down to 3/32 of an inch. New pads can range from about three-eighths to half an inch depending on the vehicle.
If you're worried that a repair shop is trying to take advantage of you by recommending brake service you don't need, first get a detailed explanation of what they say you need (ask them to show you the worn parts, as well), then get a second opinion.
Brakes wear out gradually, so you might not notice a slow but steady decline in stopping ability. A good way to keep tabs on the condition of your brakes is to have them inspected at each oil change. That might give you a heads-up that you'll need new pads or rotors within the next few months so you have time to choose where and when you have the work done.