Learn to count outs then memorize this
outs and odds chart (or
this one if you prefer).
A few quick outs totals to remember:
If you have 4 to a flush, you have 9 outs
If you have an open ended straight draw, you have 8 outs
If you have 2 overcards, you have 6 outs to make a pair
If you have a pair, you have 5 outs to 2 pair or 3 of a kind
If you have a gutshot straight, you have 4 outs
Note that not all of your outs will always be good. For example, if you have an open ended straight draw, but there is a flush draw on the board, 2 of your 8 outs will complete the flush, so if someone else is drawing to the flush, you really only have 6 outs.