Bridge is a game of catching inferences whenever, wherever, at times these become crystal clear, but sometimes they are camaflouged to deceive the declarer play into the wrong lane. Nevertheless the deductions to be made from an opponent's particular lead or move is a good declarer's prime responsibility.
It not only makes the play easier for him, but does give him the satisfaction that he is making good judgements in picking the right inference from what the opponents offer. One thing is quite certain: that no reasonable opponent would give you an undue advantage. Of course defenders do commit mistakes that are a part of the game. Everyone does. But if at any stage, you as declarer come across a line of defence that seems strange and on the surface absurdly advantageous, that is the time to be wary of it and not follow the line of play that you could have pursued without your opponent's help. Let us illustrate by two examples - the first clearly rejecting the saying 'Do not look a gift horse in the mouth' for this does not apply to Bridge.
North south are in 4S after the following auction holding these hands.
The bidding;
The opening lead is 2C. Place yourself in the south seat and see the following unusual defence unfold. East wins with QC, continues with AC both holding and then surprisingly the KC, knowing it is going for ruff and sluff.
While you ponder at this unexpected help being offered by the opponent, you survey the dummy to see two possible losers in spade if unable to trap the QS. East's ruff and Sluff manouvre on the surface seems to have improved your chances if, say, you throw a diamond from hand ruffing in dummy to later discard another diamond on the 4th heart affording even to giving up a trump trick for your contract to make if all goes well.
But the gift of the Trojan Horse is too well known for a wary declarer who can see that east could have easily exited in hearts. Here lies an inference that south needs to pick. Did you pick the same? Who has the well guarded QS? and the KD?
Well, just think that if you ruff in dummy with 10S you cannot trap the QS if it is well guarded with 3 more. So the deception in making east look too innocent clearly points out that he holds the QS heavily guarded and at the same time cannot be holding the KD, which would not lead him to this ploy.
So the correct line of play presents itself from the inference gathered. Reject the ruff and sluff and rely on the diamond finesse. Ruff the 3rd club in hand, cross to dummy with KS, run the JS. If east does not cover, take a third round of trumps in hand with ace and finesse JD successfully to play on hearts till east takes his trumps trick whenever he likes.
Sometimes your bidding tells your tale to the opponents and therefore the Trojan Horse gift offered depends on how much the opponents know about your hand, as in our second illustration where NS reach 4H after south bids 1H, north responds 2C. South jumps to 3H, north puts in 3S and south 4H.The opening lead is 5D on following NS hands:
After east took his AC and returned 9 D, when you play trump's, you find west with Q10xx. East's discards are 2 and 3S, suggesting odd no of cards, obviously 5. When west now exits with a club, do you finesse or go for a ruffing finesse is the vital question to be tackled well. This was an open and shut case as far as the clear inference is concerned.
You see west can count your distribution from partner's discards telling him that you as south hold 2 spades 7 heats, the KQX of diamonds and therefore a singleton club. Can east have the KC? No ! Because then west would have removed dummy's only entry of AS ensuring that you can not profit from a ruffing finesse. The arrow points for south to take the marked finesse of clubs. The inferences are there. You only need to catch them-whenever, wherever!

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North
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K J 10
K 10 7 4
A J 4 3
9 7
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South
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A 7 6 5 4
A Q J
Q 8 6
8 3
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North
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A 9 8 6
-
7 3 2
A Q J 6 4 2
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South
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J 4
A K J 9 8 4 2
K Q 1 0
5
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W       N     E     S
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-       -    1C    1S
2C     3C     P    4S
ALL PASS
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