Calculating your odds is VERY easy after the flop. Use the rule of 4 & 2.
If there are 2 cards to come (turn & river), then multiply the number of outs you have by 4, and that's the percentage for making your hand.
For example, you have Ac7c. The flop comes out club, club, king. Your opponent makes a pot sized bet, so you put him on top pair. Thus, your outs are an ace (3 left in the deck), or a club (9 left in the deck is usually a good assumption). So you have 12 outs. Thus, your odds of winning the hand is about 48% (the actual percentage is 44%) if your opponent has a pair of kings. Of course, this rule isn't exact, but it's usually accurate +-5%.
This simple rule works for more than just flush draws. Open ended straight draws (8 outs, 32% chance to hit), ect. all work. If you have 1 card to come, then you simply multiply by 2 instead of 4.
Of course, pot odds
come into play. Often times, even if you're a 4:1 dog, you still should call your opponent's bets if there's a ton of money in the pot already.