How to differentiate between a bangladeshi and an assamese?

I'm from Kerala and today I met a guy who is working as a staff in a restaurant, I started having a conversation and found out he's muslim and he's from Assam. I started conversing in assamese (learnt from my friends) and turns out he doesn't know the language. He doesn't even look like a northeasterner he looks more like he's from West Bengal. I want to know is there a way we can find out if he's an assamese? Should I contact the authorities and what will happen to them if they are found to bangladeshi?