There are thumb rules for the preflop equity of pocket pairs and suited-connected combos.
For the equity of a pocket pair, you calculate how many cards away from 2 your cards are (for example, Queens are 10 cards away from 2), multiply by 3 and add 50%.
So QQ's preflop equity is approximately:
(10 * 3) + 50 = 80%
For suited-connected combos (JTs, for example) the same rule applies; You calculate how many cards away from two the highest card is (Jack is 9 cards away from 2) and add 30%.
So JTs preflop equity is approximately:
(9 * 3) + 30 = 57%