The opening lead will sometimes pinpoint the leader's exact length in the suit. If it's a NT contract, for example, and your partner or an opponent leads the 2 of spades (or the lowest spot card), you'll know she has exactly 4 spades (assuming 4th-best leads). Once you know opening leader’s length you can add that to your length and dummy’s length to find out the number of cards the fourth player holds in the suit.