The gears are stamped on both the ring and pinion with the number of teeth. Ring divided by pinion = ratio.

Or use a grease pencil to mark a tooth and count 'em the hard way.

Be sure to have a new gasket, a scraper, degreaser and a can of paint handy, as well as the lube. And...just to be safe, thanks to one bad experience long ago...I usually crack loose the filler bolt BEFORE I disturb the cover.