This is an interesting topic. A friend of mine actually works in the dairy industry and consulted heavily in India. There are some clear problems with logistics. Refrigeration is not always implemented depending on locality and brand. This means that milk is fine if had fresh, but will spoil very quickly. Also, there are problems with counterfeit milk, which can even use recycled bags taken from the trash. The big storess do not have this problem, but small vendors do. These are easy to avoid, though, because the heat seal on the seams will be rough and easy to spot. Just like with all milk, always take a taste first to ensure it has not soured. Bags that feel bloated are already spoiling, causing a buildup of gas.
But do not worry too much. A lot of people forget this, but when milk spoils, it is actually turning into buttermilk, a great ingredient for pancakes and waffles. There is a bit more to the process, but getting a sip, or even drinking sour milk will not kill you. It won't be tasty, but it is not the end of the world.