General rule...
1. Count how many out you have (also note Pelions post above on taking out specific cards on flush draws unless you hold the Ace of that suit and your on the flush draw yourself.).
2. Times it by 4 on the flop, 2 on the river.
3. This equals your approx. % of improving the hand.
4. Add up what the pot size would be if you called (and whoever else would called in front of you).
5. How much% is it to call for you (i.e. $30 to call on a $100 estimated pot size at the end of the flop/turn is 30%)

To call = STEP 5 % is LESS then your STEP 2 %.

To fold = STEP 5 % is MORE then your STEP 2 %.