Make sure

Before you can even finish counting your HCP, LHO opens 3♦ as dealer and the other two pass quickly. You finish your count and find that you have 20 HCP, a balanced hand and a stopper. So you bid 3NT, and every one passes. LHO leads ♦ J and this is what you see

You play small from dummy and RHO plays the Queen. First things first, if diamonds are 6-2, you are dead. They have atleast five tricks, if you duck – 3 diamonds and 2 black aces. If you don’t duck they have seven atleast.

So you must assume that 3♦ was based on a seven carder suit. What now. If RHO has both black aces, and spades come home for 4 tricks (they must, otherwise your chances of making are really remote), you can simply win and come to 4 spades, 3 hearts, one club and one diamond trick -by winning the 1st trick.

But its not impossible for LHO to hold an ace, in which case you must duck.

Can you afford to duck? For one you’ll need to guess hearts. Plus there’s a risk that they’ll be able to setup 2 club tricks, and be able to cash 2 clubs, 2 diamonds and one spade.

Still it seems a lot riskier to win the diamond than the risk of clubs. It may not be clear to RHO to shift to clubs, or ♣ J might be singleton or doubleton.

You decide to duck, and to your (temporary) relief RHO returns the ♥ 9 (standard). There’s no hurry to try the ten, so you win ace and knock out ♠ A. LHO follows twice and then discards a diamond, while RHO ducks twice and then wins the Ace. He now returns a club, you put up the king and it holds. That marks RHO with the Ace, and there’s no reason for LHO to duck, and the position isn’t known to him.

You now cross to ♥ Q and cash the spades, discarding diamonds. LHO also discards diamonds, while RHO discards clubs. The position at this stage is

You have to get the hearts right to make your contract. Finesse or drop.

Of course, instead of guessing you make sure.

You know LHO has seven diamonds, 2 spades, atleast 2 hearts and atleast one club. His last unknown card is either a small heart or ♥ J or a club. 

Similary RHO is now down to either two hearts and two clubs including the Ace, or three clubs and one heart. 

So you play a club off the table. Worst case scenario he can cash ♣ A and ♣ J, then he must play a ♥ . Meanwhile, if you see RHO discard on the 2nd club, you’ll know to play for a drop in hearts, while if he follows, the finesse is a certainty.

Feels good to avoid the guess, doesn’t it!

The full hand: