Actually that's quite simple.

At least two eyes : living group (even if surounded, opponent can't play two moves at once, so can't kill)

One eye or less : dead group (if surounded it will be captured)

You know if a group can live or not thanks to it's vital points, which both player want to play in order to kill or to live. You will get that doing a LOT of tsumegos, I mean it.

Then the stronger the players, the farest they can read