If calliper disc brakes:

What you'll need to do the job:
Pliers, Allen wrenches or flat head screwdriver. New pads.

Disc brakes are easy to check because you can see the brake pad without touching anything. If the pad has worn down through the asbestos into the plate, it's time to change your brake pads.

To change pads, remove any pins, clips or springs from the calliper, and remove pads. Insert new pads; replace pins, clips and springs. Check that calliper bolts are hand tight and secure. Ride the bike and check the brakes.

Replacing or checking drum brakes requires removing the wheel.