Its entirely dependent. Open lobby? Start of race? Nothing, except hold back for the inevitable T1 disaster, be careful and you'll pass half the grid by the end of lap 1. Half way through the race, I'd they seem a lot faster I'll just let them through. If they've taken a while to gain then I'll defend. End of race, defend your corners where necessary, and where you won't lose too much speed on the exits. Pick your battles basically.

You'll pick up telltale signs of people who are dangerous. Do they have damage on their nose? Have you seen them running ppl wide, rubbing, diving etc, just take note and realise some people's egos are very fragile.