The sound that your brakes or brake pads make when they are going out are sounds such as screeching, squealing or grinding.
When you press on your brake pedal and you hear sounds such as screeching, squealing or grinding such as metal on metal then your brake pads could be totally worn down.
Your brake pads also have a wear indicator on them that scrapes against the brake rotors when the brake pads get worn down enough.
When you hear a screeching or even scraping down when pressing on your brake pedal then it is time to look at the brake pads to see how far they are worn down.
Also if your brake pedal is going to the floor more then your brake pads could also be worn down and need replaced.
Or if you completely lose your brakes then your brake pedal will go all the way to the floor or at least partially to the floor and it may be hard if not impossible to stop the vehicle.
Look for signs of leaking brake wheel cylinders if you have rear brake drums and if they are leaking the wheel cylinders need replaced.