[Will C]

The quick and dirty way is to determine the number of outs you have to win the hand. The number of outs you hold, divided by 24 will roughly give you the percentage you have of winning the pot (if you think about it, you'll see why this is approximately true).

For example, say you hold Ac5c, the board is Kc10s3c, and you have put him on KJ or KQ. Thus you are fairly certain that any ace, as well as any club would likely make you the winner. Your outs are 3 aces and 9 clubs, or 12 outs. 12 divided by 24 says you should roughly have a 50% chance to win the hand, which is approximately correct. When I throw the numbers into pokerstove with holdings of Ac5c vs KhQs with a board of Kc10s3c, I get the Ac5c having an equity of ~45.6%. Hope that helps.