Source: lajollabridge.com 
When counting out opponents’ hands, there are two important numbers to remember: 6 and 12/13. Six is the number of HCP with which a player will almost always respond. Therefore, if one opponent opens and the other opponent passes, and you get the contract, play the opponent who passed for less than 6 HCP. (If, for example, that opponent shows up with a King early on, you know that the other opponent should have all other unseen Aces and Kings.)
Twelve/thirteen is the HCP with which a player will almost always open. If an opponent passes originally and shows up with 9 HCP in the play of the hand, it is almost certain that any other Ace is in the opposite opponent’s hand. Probably any other King is in that opposite opponent’s hand as well.
If an opponent has opened 1NT and your side buys the contract, you should know almost exactly what the NT bidder’s partner can have. Suppose, for example, you overcall 2 with your:
AQJ109x 1098 xx xx and dummy comes down with
8x KJx Axxx KJx. You have 7 HCP and dummy has 12 HCP for a total of 19 HCP. The NT bidder (on your right) has 15-17 HCP. That leaves 4 to 6 points in the hand of LHO. If LHO leads the queen of diamonds, you know that s/he has QJ of diamonds (3 HCP), so CANNOT have any Ace and probably does not have any King in his/her hand.
So, you expect that the spade finesse will work, but you expect to lose 1 diamond, 2 hearts (if RHO has queen as well as Ace) and 2 clubs (if RHO has queen as well as Ace). Your best bet, after taking care of trumps, is to play the 10 and let it ride. If LHO has the queen of hearts, you will actually make 9 tricks. If RHO has both Ace & Queen of hearts, you can try a low club to the jack—in case LHO has queen of clubs. If so, you take 9 tricks. If not, you take 8. (If the spade finesses loses, expect to go down one because RHO must have all the other high cards!)
Look for what your opponents did NOT lead! Remember, people will make the best lead possible (usually) and top of a sequence is an excellent lead. So, if your side is missing the Ace, King, and Queen in the same suit, and the opening leader does NOT lead that suit, assume that s/he does NOT have AK OR KQ in that suit. Depending on the rest of the hand, you may place LHO with AQx or Kxx or nothing in that suit.
When the opponents have competed by bidding and raising a suit, but do NOT lead that suit, it is likely that either: (1) LHO has the Ace of that suit and does not want to just lay it down; (2) LHO is leading a singleton hoping to reach partner in that suit and get a ruff; (3) LHO has another card combination which is an excellent lead.
When the bidding screams for a trump lead (e.g., you have bid two suits and dummy takes a preference) and LHO does NOT lead a trump and makes some other lead that is rather lackluster (not from a strong sequence), assume that LHO has a vulnerable trump holding from which s/he does not want to lead (e.g., Qxx or Kx).
Use discovery plays. Often, you need to “force out” an honor in one suit (e.g., Ace of hearts) in order to decide how to play another suit (whether to finesse for the King of trumps or try to drop it singleton off-sides). Suppose LHO has opened 1, you buy the bid for 3 and LHO leads the Ace, King, then Queen of diamonds, where you trump third round. Assume your hand is:
AQJ1098 Kxx xx Qx opposite
7654 QJx xxx AKx. You have 12 HCP and dummy has 10 for a total of 22. LHO probably has 12-14 and RHO 4-6 HCP. So, RHO can have the A OR the K—but not both. Force out the Ace of hearts first. If RHO has it, forget the trump finesse. You KNOW that LHO has the King. Play the Ace and hope the King is singleton.
Use necessary assumptions. If you must have a certain card in a particular opponent’s hand in order to make your contract, assume that it is there. Placing that card may allow you to place other cards. (If they are not there, you cannot make it anyway, so your best approach is to start with the assumption that the cards are where you need them to be.)