The other day I was travelling in a bus with a Sinhalese friend of mine. On the seats in front of us there was a Tamil girl who was speaking on her phone. Within hearing the first few words she spoke I told my friend that she was from the from the hill country pretending as a Jaffna Tamil on the call. "He asked do you know her?", I replied "No". He didn't buy in my answer and insisted I knew her and trying to cover it up, he demanded an explanation on how I was so sure. I tried to explain to him by saying that its the nature of Tamil language and we have different dialects with sharp differences. So at a blink you could say the person's region. He was asking me to explain the exact phrases which should be taken into consideration to differentiate.
So I tried to to explain by saying that you need to consider the pronunciation, the terminology used, the influence of other language words, etc When you consider all these factors and when considered in sync by an experienced Tamil language speaker he can distinguish it easily. But if you want me to explain how I differentiated this its impossible since I am not an expert in linguistics. He was not still accepting the story so I picked an example from the book blink, and explained to him why there is a unique profession called as tea testers, all of us can drink tea and tell whether if it tastes good or not, but it takes only an expert to explain why a certain tea is better than the other. He would consider different factors like the colour, smell, texture, etc and then make a rational judgement and will be in a position to explain the rationale behind his judgement.
So the beauty of Tamil language is when people without knowing the finer points of the dialect try to pretend it doesn't take rocket science to figure it out! And if you are a linguistic expert you can actually prove the fact with a perfect reasoning. I wish that girl makes a better attempt next time around so she gets it better than this, so she can at least fool a non-expert ! Good Luck!