Reading opponents cards is not exclusively the domain of the experts. You and I can also venture to do the same with some success only if we keep our thinking cap on and stay focused to what the bidding of play of the opponent's cards tells. Every now and then you will come up with a hand that requires quite a few assumptions. If you are attuned properly to the happenings at the Bridge table, you will succeed with consummate ease so as to give you great contentment and satisfaction at your reasoning.
Before giving you the illustration for today, let us discuss a hand that was played many years back by one of America's leading expert and it is for you to learn the intricacies of assumptions in Bridge from the way this expert handled the following contract:
The bidding went as under:
Against this contract west led the inevitable small spade - the deuce. As south, how would you have tackled this contract of 3NT, especially the first trick which was crucial for the contract. It is a hard choice - the king placing west with the ace or letting it ride to the jack placing east with the ace and west with the queen. A lot of thoughts crowd as to the length of the spade division and its relevance for the contract of 3NT to make. The American expert reasoned thus: "For me to make 3NT I need winning finesses in clubs and diamonds which would place east with both minor kings. Assuming a 4 carder spade lead as apparent from the deuce led by west, this places east with 5 spades on which east still failed to overcall. Thus east could not hold the AS". So he played KS at trick 1 and was duly rewarded when east turned up with both minor kings.
Similarly Jim Jacoby another expert faced a similar problem when he bid 5H after the opponent's competed upto 4S on the following hand:
On the opening lead of spade 3 Jim had to decide whether it was from the king or the jack. Based on his reasoning that the contract cannot be made if west has either of the minor suit aces. So placing east with both aces and his failure to over call Jim placed the KS with west and played the QS correctly when east turned up with both minor aces.
We now come to our main illustration on the same theme with the following hand:
West opened 1NT (16-18). North bid Landy 2C for majors and on 2D from east, south ventured into 4S. West led KH, east signaled the deuce. West cashed AD and east again signaled the same. West now shifted to 3S. As south how would you play assuming that east could well hold 4 points which could be an ace? Yet the contract is making despite the 3 missing aces if you assume logically that east must hold the ace of clubs to enable you to make the contract as he could not be holding any other point. Hence west has led from the QJ of spades which should be your correct assumption. And playing 10S at trick one is what the bidding tells us.

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North         South
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  Q10            A7
 AKQJ      10876532
   97            K5
J8765            K2
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North         South
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   K7            J4
 AK42           853
10965          AQJ4
 J108          AQ97
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E    S    W       N
P   1D    P      1H
P   1NT   P     2NT
P   3NT    ALL PASS
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North         South
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AK104         98762
Q6543             J
   KQ            J9
   J2         K8764
===================