the best way to be sure its not a fake is to go to a caltex supplier or depot and buy it from them, In Khi, mohmand oil trading in Lines area have it by the drum, they also change your cars engine oil for Rs 30 or something if you want, you can also choose to have them pour from the drum or from a 4 litre bottle pack. Pouring from the drum is best, its cheaper than the bottle, its the most latest stock and you can buy it in exact quantities you want with no left over.
They sell more oil in a day than a parts shop would sell in 2 months.And I used to buy 210 litre drums from them too.
As the fake oils are all packaged in used original bottles there is no way to distinguish other than taking it to a lab.
Siimilarly mobil and PSO also have a similar depot at the Truckstop when going to Hawkesbay,