If you play a lot of sit & Go's at places like Party Poker, like I do, you will realize that towards the end of a tournament most of the decision making occurs before the flop. However, poker players are notoriously bad at working out when to call their opponents all-in bets.
Working out whether you should call or not should be the easiest decision to make in poker, because you don't have to consider subsequent betting rounds. It is a simple matter of working out the pot odds and estimating your chance of winning and then comparing the two.
Pot oddsPot odds are very easy to calculate, as it is an exact science with no deduction required. Simply sum what's in the pot to what your opponent bet and then divide by your opponents bet. For example, blinds are $400/$800 and your opponent has bet $2000 all-in. Pot odds are ($400+$800+$2000)/$2000 = 1.6 to 1. So if you have your opponent covered and are better than 1.6/1 to win then you should call. Note, if you don't have your opponent covered you should divide by your chip stack rather than your opponents bet. Easy huh!
Chance of winningIf you think calculating pot odds was easy, well you are in for a bit of a shock. Calculating your percentage chance of winning is not easy. Thankfully, there are a few tricks to help you. First you need to know a few things. You need to know that there are 6 combinations that make up any pair and 16 combinations that make up any two given cards. So there are 6 combinations that make up 10,10 and 16 combinations that make up, say AK. This of course assumes suited and unsuited hands represent the same value. It is too hard to factor in suits without simulation software.
Ok, that makes our task a lot easier. Say we hold 99 and we think our opponent either has an over pair or AQ or AK. This would be a tight opponent granted, but this could be more than possible. There are 30 combinations of over pairs (TT,JJ,QQ,KK,AA)=5*6 and there are 32 combinations of overcards (AK,AQ)=2*16. So it is close to 50/50 that your opponent is holding an overpair vs. overcards. Not too hard so far? You can do these types of calculations for any given situation. Now comes the tricky bit. You need to know what your chance of winning is for each of these scenarios. Simulation software is critical here. A program like PokerWiz will do the trick. I can tell you that your chance of winning when facing two overcards is around 50% (slightly less but we don't need to be exact). You chance of winning facing an overpair is around 20%.
30*0.5 = 15
32*.2 = 6.4
Total = 21.4
Chance of winning = 21.4/62 which is close to 3/1. Given the above example, you are not getting sufficient pot odds to call. However, if your opponent was slightly looser and would make the same move with KQ or 88 or 77 or 66, then clearly the calculations would change a lot.
CombinationsOvercards = 48
Overpocket = 32
Underpocket = 48
Total combinations = 118
Overcards = 48*0.5 = 24
Overpocket = 32*.2 = 6.4
Underpocket = 48*0.8 = 38.4
total winning combinations = 68.8
Odds 1/(68.8/118) = 1.72/1 which is still not quiet enough to call but much closer.
It is more of an art that a science to calculate our chance of winning. Furthermore, you need to memorize your chance of winning for given situations. A few you should know include, over cards, one overcard and one under card, two undercards, over pocket pairs, under pocket pairs etc. But you need to do the work to help your understanding, I have just provided the framework. A good reference on this is Dan Harrington's two books series:
Harrington on Hold'em: Expert Strategy For No-Limit Tournaments (Volumes 1 and 2).
Volume 2 in particular has a lot of stuff on these sort of calculations.