Opening leads often tell you a lot about the defenders’ hands.
Against a NT Contract:
1) Use rule of 11 in cases where the Opening Leader appears to have made a 4th-best lead. (Subtract the spot card led from 11. The answer is the number of cards above that spot card in your hand, dummy’s hand, and RHO’s hand.)
2) If Opening Leader makes a “top of nothing” lead, assume that s/he has length in a suit you have bid, or a suit dummy has bid. If neither of you has bid a suit, assume Opening Leader has tenaces or honors in the other three suits that s/he did not wish to lead away from.
3) If RHO has bid a suit and you bid NT and LHO does NOT lead that suit, suspect that LHO is void or has the Ace in that suit (unless LHO has a clear-cut, good lead such as top of a sequence in another suit).
4) If Opening Leader leads a suit that you or dummy have bid, assume that s/he probably has 5 (or more) cards in the suit.
Against a Suit Contract:
1) If the opponents have bid and supported a suit and the Opening Leader makes a lack-luster lead in another suit, or leads trump, assume that the Opening Leader has the Ace in the suit they have bid.
2) If the opponents have bid and supported a suit and the opening lead is a suit in which you or dummy have length, suspect that it might be a singleton.
3) If the opening lead looks like the top of a doubleton, plan your play accordingly.
4) If you and your partner are lacking the Ace, King and Queen in a side suit and it is NOT led, assume that either they are all three on your right or that LHO has a vulnerable holding such as AQx in that suit.
5) When you bid 2 suits and dummy takes a preference, and the opening lead is a trump, LHO may be well upholstered in your other suit.
6) When an opponent leads trump, s/he often has scattered honors in the other three suits which s/he does not want to lead away from.
7) When the auction calls for a trump lead and LHO does NOT lead a trump, assume a vulnerable trump holding (such as Qxx or Kx).
Know what different cards promise and watch your opponents’ signals. (Check their convention card or ask to see if they are playing anything nonstandard in terms of signals.) Assume that your opponents are reasonable players unless/until proven otherwise. If, for example, you have two small cards in a suit and dummy has Jxx, and LHO leads the Ace (promising the King) and RHO signals high, assume that RHO has the queen in that suit.
It is rare for a player to lead queen from queen doubleton (although it does happen occasionally)—unless partner has bid the suit. If an opponent leads the queen in a side suit and you have Axx in your hand opposite K10xx in dummy, assume that LHO has the jack. Take your Ace and finesse the 10 next time you play that suit.
Remember the bidding when counting opponents’ high card points. Someone who passed originally will NOT show up with 13 HCP. Usually, s/he will not even have 12 HCP. Thus, if you have seen 10 HCP from that hand already, put any other Aces and Kings—and probably queens as well—in the opposite opponent’s hand.
Do NOT make unilateral plays. Preserve your options! For example, someone leads low (generally promising an honor) and you have KJ4 in dummy opposite A83 in your hand. Do NOT play the Jack. Play low. Sometimes, RHO will play the queen. If RHO plays the 9 or 10, you have time to try the Jack later. It won’t go away!!