Believe it or not, most of the time you can check pad wear without taking off the wheels. And you don't need a mechanical engineering degree to do it. Usually, you can see the brake pad through the wheel and won't need to remove it. Once you find the brake pad, notice its thickness. If it appears to be very thin, it's almost used up. Some brake pads have a slot in the center that serves several engineering purposes, but also doubles as a wear indicator. Check to see how much of that slot is left. If it's almost gone, you need new pads [source: CDX eTextbook].
In some instances, you may need to remove the wheel but can inspect your pads through an inspection opening on the caliper itself. This is a small window that gives you a cross-section view of the rotor and pads. The less material you have left, the closer you are to needing new ones. (You can see a diagram of a disc brake here.) With the wheel removed, you should be able to determine the pad's thickness pretty well. If you want to take it a step further and inspect the condition of the pad itself, you'll need to remove the caliper from the rotor. If you've never done this before, pick up an automotive repair manual, or take the car to a mechanic.
Brake dust is the most obvious sign of brake wear. The heavier the car, the more brake dust you'll see on the front wheels versus the rear. If you start to notice less brake dust, that's a sign that you may have worn your brake pads down to the metal backing.
Your ears can help determine brake pad wear as well. If you hear a screeching metallic noise when you hit the brakes, it could be the wear indicator. Wear indicators are small metal tabs designed to come into contact with the rotor once the pad wears to a certain point. Wear indicators are great in that they give you a heads up that it's time for a change before the pad wears too thin [source: CDX eTextbook].
So far we've talked about how sight and sound can help detect brake wear. Let's look at how feeling can help too in the next section.