Many new model cars have dashboard warning lights indicating that the brakes are worn. If not, you may be able to see how much friction material is left on the pad through a large inspection hole in the side of the caliper. Many pads are made with a deep groove down the center of the friction material. As the material wears, the groove disappears. Otherwise, you'll have to look at the edge of the caliper to see the edge of the brake pads.
Many cars now have an audible warning in the form of a metal tab in the brake pad.
If the pad's worn to its safe limit, the tab will hit the rotor each time you step on the brakes and you'll hear an awful shriek from the brakes each time you use them. Besides noting how much friction material is left on the surface of the brake pads, it's also important to note that each pad is wearing at the same rate as its mates. If only one pad on a specific wheel is wearing, it's a sign of that the brakes are not operating properly and must be checked by a pro. Next, inspect all the hoses, steel tubes and all of the connections. Check the hoses for signs of chafing, brittleness, softness and cracks.