If you think the gears were not changed yoy can look for the letter stamp on the axle shaft that gives you the axle ratio. For the Model 20 rear end look on the pass side of the diff. You may have to clean and remove some crud but on the rear axle on the side that the cover is on the factory stamped a letter (example: A, BB, D, DD) that is the ratio for that axle. If you can't find it you could pop off the cover count the teeth or get the ratio of the gears themselves. Or you could do the math as foggy said. Or finally when the police comes to your window and says "you were do x in a 55."
I vote for either option one or two first. Goodluck.