if there on the ground and disassembled i would look in the chopper port on the chopper cap , if the horn is still on it shine a light down the throat of the horn. if you can visually see in the chopper slot look to see if it doesnt have a dividend in the magnesium chopper assembly, if not and it has a large port then its single tone. if it has a dividend in the chopper it self and two seperate ports then its a duel tone. Like everyone else said if its still assembled and on a pole and still hooked to power bump the chopper just a bit in the rcm panel to see if it is single or duel tone.
these are pictures from my collection of some 3 phase siren stuff that came from around the cincinnati area and what you might run into.
here is what a mid 1950's 3 phase RCM panel looks like..
here is what a 3 phase 1950s A1 blower with 3 phase motor looks like
here is what a 1960s 5m blower with 3 phase motor looks like, blower needs to be degreased and not sure of motor brand.
Female enthusiast who is the owner of many large and small sirens, misc controls, timers etc. (too many to list in this small space) all big sirens came from cincinnatis old system.