tdracer is absolutely correct. We did this exercise in the Sim for initial training on the 737 Classic -400. I cannot speak to the NG and how the following issues relate but in a simulator at night, as most sim sessions are, the lights suddenly go out and there is a very quick way to determine you have generator failure or engine failure. Look at the top of the instrument stack and see if you have oil pressure. The Low Oil pressure lights are powered off the battery bus, if they are out then chances are you have a black cockpit due to a generator failure. If you are staring at two illuminated amber low oil pressure lights you have a dual flame out.
I don't know about the configuration of the NG. However it easy, at least initially, to mistake a dual generator failure for a dual flame out.