[angelxmagic]

I had to google it, but I think I understand now. But how do I recognise a southern Indian? (Btw, does that mean a Dravidic person, not an Indo-Aryan?) Can a name help?

Generally, yes, south Indian means of Dravidian extraction. Southern names are quite often distinctive, as you pointed out below.

Some Indians here have "stereotypical" long names with easily identifiable roots, like Rajasekhar or Mahalingam .

Those are the stereotypically South Indian names that I'm talking about, but you can never be sure.

Some only have a short name with initials, like P J Harvey (name changed to protect the innocent). Maybe their accents or skin tone can be of any use?

The accent is something which helps me distinguish, but it can be hard to know for someone who doesn't know the nuances of linguistic variation in India. For instance, the difference between a Punjabi and a Tamil accent is glaringly obvious to someone from like me, from Maharashtra, but it may not be to someone who isn't familiar with them.

The skin tone is a definite indicator, but the problem is that you'll find that it correlates quite often to caste and economic background as much as position on the North-South axis. As a general rule, the more you move towards the South, the darker the skin gets.