I haven't worked for my current employer long enough to know whether it's great or merely good, but it sure looks promising.

The main thing I look for is morale. If morale is high, in both managers and general employees, if people smile and say hello to each other in the hallways, if requests for help are handled cheerfully and competently, and so on, then I think it's a great company. (Of course, it also has to have a business model that actually works, has to produce a high quality product/service for a fair price, etc. That the baseline.)