Many beginner books on hold 'em are very good at explaining and introducing how to calculate odds (I used Ken Warren's book) but for a quick reference in the heat of battle.

Figure out how many cards could hit on the turn or river that would give you a hand to beat the highest reasonable hand from the board, depending upon your read of the other hands.

For the turn or a one card river, multiply the number of outs times 2.5. (This is not exact but close enough with implied odds and easy to use).
For two cards, multiply the number of outs times 5.

When you get the number you can calculate if a call is in order. (ex. 7 outs x 5 = 35%. for you to improve on the turn or river if taking two cards; you need about 1 to 3 on your money to call).