I use a common method. Let's say that they have AK so there are 6 cards (called Outs) still in the deck of 47 (after the flop). Since there are 3 cards in the flop, they have 3x6=18 chances to hit an Out. That means 47 / (3x6 Outs) ~ 2.5 (ie. once every 2.5 hands).
If they miss on the Flop, then the odds of getting a pair on the Turn is 46 / 6 ~ 8 (ie. 1 in 8). Similarly for the River.
Your chances of hitting trips is roughly 47 / (3x2 Outs) ~ 8 (ie. about one in Eight). If you miss your trips, then 46 / 2 ~ 23 (ie. about 1 chance in 23) on the Turn. Usually if I play small pairs and miss hitting trips, then I will fold if there is a bet.