A common emitter amplifier does just what it says. It amplifies a signal. Someone tell me if i'm wrong here, but a common emitter amplifier is usually used to amplify a varying signal. The cool thing about it is that it can amplify an AC signal that swings from negative to positive. There are different kinds of amplifiers -common emitter, common base, and common collector, and I don't totally understand all of these accept for the common emitter. The common emitter is the simplest (for me anyway). If you post a picture of one, I or someone else will tell you how it works. As for your question on how to test one, I'll leave that up to someone else.