and opponent's range. So you figure out how often you need to be good based on the odds you're getting to call and then you guess based on your opponents actions the hands he has in his range. Figure out how much of that range you beat and call when you beat enough of it to make calling +EV.
Simple non real world example:
Pot is $10 on river and your opponent bets $10.
The board is AJ379.
You have KK.
You somehow know your opponent has either QJ/JT/AQ/AK.
Should you call?
Well you need to be good 33% of the time to break even (2:1 or 10/30) and against the range of hands our opponent could have we're actually going to be good greater than 50% of the time (we beat 24 of 45 combos), so we should call.