Not in yet, but I would look to see how professional the interviewer acts towards you, and try to get a few ride a longs on, and see if you like the way they act "when the boss isn't around." But the thing about shiny ambulances isn't really a joke... Make sure they do all the maintenance and up keep on the rigs, if the rigs don't look good, how do you think they will treat you?
I would say that the best thing is to do some research. Ask local EMT's that may have worked for a particular ambulance company....see if they can tell you how they treat their employees and what the average starting pay is so that you have some idea, and so forth.