How do the police tell how big a scooter engine is?

now i just down graded my motorized bike to be road legal in Maine i went from a 49cc to a 24cc engine (after my friend got pulled over and almost arrested) in maine the law is it cant be over 25 cc (the maine dmv on scooters http://www.maine.gov/sos/bmv/registration/scooter.html ) but im afraid im gonna get pulled over and the cop wont believe me thats its only 24cc how can he find out its only 24cc (its a little china motor with no markings) Thanks