Jet size is usually a good indicator, you could open up the carb and check to see what jet size you're running.
I'm pretty sure they went #48 for the 20mph, #50 for the 25, and #52 for the 30
Also, go to the moped wiki and search for garelli, there's info in there about the "yellow dot" slow cylinder vs. The "red dot" fast one, and how to tell which one you have.