On some vehicles, you can see the disc brakes without taking the wheels off. If you can't see the disc rotor or drums, then start by taking one wheel from the front and one from the back off.
With disc brakes, you'll see the disc, pistons, and other brake hardware exposed. The drum brakes will have a "bowl-like" cover over the brake hardware. This "bowl" is the brake drum.
picture of disc brake:
http://static.howstuffworks.com/gif/disc-brake4.jpg
picture of drum brake:
http://static.howstuffworks.com/gif/drum-brake2.jpg