How do I know if a generator head is putting out AC or DC?

Generally you can tell AC from DC circuits by the outlet configuration but don't depend on it totally. Look at your wall outlets, they are probably 115vac (volts alternating current). There are usually two knife and one rounded opening in each outlet. A 12vdc (volts direct current) circuit outlet will have two round sockets usually marked red and black or + and -. The generator will be marked as to what voltage and type of current it puts out.