If you look at the back of the brake caliper, you'll see the tabs at the back end of the brake pads sticking out, with a pin through them from one side of the caliper to the other.
The gap between them is an indicator of how much brake pad you have left.
e.g if the gap is 10mm, take off 6mm for the thickness of the rotor (disc) and then divide by two to get the thickness of each brake pad material - in this case 2mm. The minimum spec is 1.5mm (some manuals say 1.8mm) so in this example the pads are near to replacement time.
If you've only got 7 or 8mm there, one pad is probably down to the metal. This will make some noise, and be pretty useless at actually braking. So always give a quick glance at that gap before riding to check you've good to go.
You can also get brake squeal from the pads vibrating at high frequency when rubbing against the rotor (disc). You can buy pads with special anti-squeal shims on them, or even make you own. Often just coating the back of the pads with a smear of anti-seize will stop the squeal. Just don't get any on the front of the pads!
Make your own shims:
Home-made anti-squeal brake shims? They work!