When your brake pads are worn out slightly then it may take a little long to come to a complete stop and the break pedal may go to the floor more.
If the brake pads are worn down enough to need replaced there will be a metal on metal scratching screeching sound when you press on the brake pedal to stop the vehicle.
Brake pads have wear indicators on them that rub against the brake rotors when they are worn down enough and when you hear that sound it's time to replace the brake pads.
You may also hear a metal on metal sound like a screeching sound or grinding noise when you press the brake pads and stop the vehicle.
If you're worried that the brake pads are worn down enough you can remove the wheels and look at them or have a brake shop look at them and inspect the brakes.
Some brake shops will inspect your brakes for free in hopes of getting your business.