It's a difficult thing unless you have access to their official/historical documents. Family name can be a starting point. But it's not enough to full ascertain

It's a difficult thing unless you have access to their official/historical documents. Family name can be a starting point. But it's not enough to full ascertain someone's caste. One thing you can do is to avoid using food preferences and skin color to figure out the caste. These have only led to more anecdotes and distract attention from the actual discrimination.