when im in doubt of whether it is an m next to me ill check the wheels (lots of people have the m wheels) rear diffuser (not so common) front bumper (common) side skirts (some forget to get em...) mirrors are a pretty good give away, check to see if it has the m3 exhuast (lots upgrade to this) another good way is to toy with him a little.... if he takes off and is faster than say your 325... theres a chance it might be real. If you arent driving by them... check as oldskool said control arm (i didnt know this one) or you can check the whole interior (expensive to swap) or run avin check?
david