How can I tell if my bike has a freewheel or a cassette? I bought a Frankenbike off Craigslist. 



It's a Specialized Rockhopper of unknown year, converted to a commuter, with shocks up front and V-brakes. 

I love this weird little bike and want to put a Copenhagen wheel on it when they come out, to make it into the ultimate commuter frankenbike. 

To install the wheel I'll need to know whether I'm dealing with a freewheel or a cassette. How can I find out without taking the bike apart or knowing the year it was built?
If you take the back wheel off you should be able to examine the freewheel/cassette to determine which it is. Here is a picture of some common types. If yours doesn't look like one in the picture, post a close up and I'm sure someone here can identify it.