It depends (sorry). Reads, stats, stacks, but most importantly consider the action from preflop until whatever point you are at. Give opp a range, consider how frequently he is bluffing, are there missed draws etc. and how often the card he is betting has hit his range. For instance, if you decide to c/c (edit: with QQ) on a Kxx board vs. an aggressive player, then the turn is an A, even if he was bluffing on the flop a lot of his bluffs now beat you.

It's sooooo situationally dependant I hope this kind of makes sense...