Squealing disc brakes are not always indicative of a need for replacement. If you’ve just washed your car, and/or there is rust on the brake rotors from lack of use, a little squeal for your first few stops is probably OK. But it should go away after several minutes of driving and brake applications.
Beyond that, a constant squeal or squeak might mean that you need to get those brakes checked out. Mechanical wear indicators (a little metal doohickey on the brake pad) make noise to remind drivers that their pads need checking, and many cars these days have electrical wear indicators that cause a dash light to come on when the pads need replacing. In either case, it is a good idea to stop by the garage and have your mechanic look it over, and see if the pads (or brake shoes) are at their “wear limit” or minimum thickness.
If you want to do a visual inspection, you should see about ¼” of pad material. This can be done be peering between the spokes of your wheels (use a flashlight) and/or removing the wheel to look and take real measurements (best way). Brake rotors also wear down, and your mechanic can check their thickness, and help you decide whether they should be resurfaced or replaced. This last bit is important – and you’re not being scammed if your mechanic suggests this – because new brake pads need a clean, uniform surface on the rotor (or drum) to clamp against. ~Bill