If you see Indians in KFC asking for vegetarian choices, you can guess that they are upper caste. Alternatively, if you see Indians speak of animal rights, while wearing leather bags/ belts or shoes you could also guess from that.

Apparently (from a cursory Google search) it seems that members of some castes (the higher ones, apparently? maybe just the highest?) are typically vegetarian.

Not necessarily. The so called warrior (and "upper") caste members have been eating meat for centuries. Brahmins (the priests) are supposed to be vegetarian. They are also supposed follow few other rules. These "sacrifices" allow them in forming better connection with gods.

'Vegetarian' diet in according to Indian system the regular vegetarian diet+ dairy. They drink milk and may consume other dairy products like curd and ghee. Some Brahmins do not eat garlic and onion.