Thats true, just now outside my house i saw a black BMW 5 series
coming towards me, then as it got closer i noticed the front and
the bumper was like that of an M5 and i thought wah lao, must be
M5, then as it went past i noticed it had the same side vents like
M5 and got excited because I've never seen real one here, then when
I saw the back i noticed it had no M5 logo 
Only way to tell some cars like that isn't real M series is to look
at the back like I did, but even sometimes the replica cars have
stuck on imitation M badges. Unless maybe I saw a de-badged real
M5, but i doubt it