In the bidding where Partner freely bids over West it tells you that his diamond suit is good

Count the points. You have 18, Partner has 8, East opened so should have at least 11. This leaves West holding just 3 points.

West bid a weak 2S, announced as weak, so should hold 6 of the suit and the lead of QS (2 points) implies that his one remaining point is JS. 

Count the spade cards. You hold 4, Partner has shown 2, if West holds 6 then East's 6S played must be a singleton.

Furthermore, you know that East holds all the remaining points, particularly, KH, JH, KD, KC and JC.

Therefore, you know that the club finesse is good but a diamond finesse will fail.

East's distribution must be either 1,3,4,5 or 1,4,4,4. Holding 4 hearts you might expect East to open 1H but equally you would not expect West to hold both majors.

Counting winners, you have 2 spades, 1 heart, 1 diamond and 2 clubs (including the QC finesse) = 6 tricks, you need 3 more tricks and the most likely source is diamonds.

The Play:

Win the opening trick with AS and start on diamonds. Lead JD, East will see the danger of Declarer establishing the diamonds so will probably duck. Continue with diamonds, say winning AD and continuing until East wins his KD. If East leads a heart the Declarer wins QH cashes his winning diamonds and make his contract. So East will lead a club. East leads a small club, Declarer wins QC and cashes AC and returns a small club, East can take his 3 winners but must then lead hearts - game over. Nine tricks, 2S, 2H, 3D and 2C.