Once the flop has been dealt in Texas Hold'em, you'll be able to count your outs and know how likely it is your hand will improve -- and that'll tell you whether you should stay in the hand or fold.
You can figure out your outs and odds for any hand, but here is a quick and dirty list of the most common scenarios:
If after the flop, you have...
Two outs -- your odds are 11 to 1 -- common scenario: hoping your pair becomes three-of-a-kind
Four outs-- your odds are 5 to 1 -- common scenario: trying to hit an inside straight draw
Eight outs -- your odds are 2 to 1-- common scenario: you have an open-ended straight draw
Nine outs -- your odds are 2 to 1 -- common scenario: you have a flush draw
Fifteen outs -- your odds are 1 to 1
