There is a nice device called Kill A Watt™ which plugs into the wall outlet or power strip, and  then you plug in the device you want to test (be it your computer, a light lamp, or any other appliance) and it will tell you how many amps or watts  the device is drawing.

 

Check it out at http://www.thinkgeek.com/gadgets/travelpower/7657/ or search for “kill a watt” using your favorite search engine.

 

Special thanks to Richard Carpenter who brought this to my attention!