How can we know whether it आईएस a बीएस6 or a bs4?

If you are purchasing a new car then you will get the BS6 version only otherwise the car will not get registered. And if you are purchasing a new car from the showroom then after your purchase you will get all the documents cleared, you get an RC smart card. On that, you can check if the car is BS6 compliant or BS4 compliant.