How to know if the engine of my 50cc 4 stroke scooter is a GY6 ( 139QMB ) or a V2 ( 137QMB ) ?

To check, you have just to look at the output of the exhaust:
– It is directed down ( perpendicular to the ground ) , it is the GY6
– It is located on the side ( lateral ) , it is V2

In addition the engine number will give you a hint : the figure 39 (= GY6) or 37 (V2) must be included