This is the test print I use for ensuring a level print bed for my MakerBot Thing-O-Matic and MakerBot Replicator.

I've included my basic directions for printing the test piece below. It's most important to watch the first layer as it is being extruded. The nozzle needs to be close enough to the print bed that the extrusion adheres to the print bed, but not so close that the extrusion isn't too thin.