I would just try it on each voltage battery. IF you don't have a 6 volt battery, then connect it to a 12 volt and if it screams way too fast, it is a 6 volt horn. If it doesn't blow on a 6 volt battery try adjusting the screw, as it may be screwed in too tight. On an original horn a click or two can make a big difference.
An original Model A 6 volt horn uses about 5 amps when correctly adjusted. My 29 Tudor had a 6 volt chrome JC Whitney horn when I bought the car. That horn actually had an excellent sound, but I sold it and put the correct horn on the car.