You start with two spades, one heart and one club, so it looks like you need five diamond tricks which requires a 2–2 division of the suit. Should you, therefore, just bang down the ♦
A and see what happens?
No. There’s a better line and it requires taking the heart finesse. Win the spade return in your hand and play a heart to the queen. If it loses, you’re no worse off than you were before: Win the return and play on diamonds.
If the finesse wins, however, you will then need only four diamond tricks, so you can afford a simple safety play. Play a low diamond from dummy, intending to insert the 7 if East follows low (the 5). If West wins this trick, you’ll know the suit was either 2–2 or 3–1 originally, and you will be able to run diamonds as soon as you regain the lead.
If East covers the low diamond exit from dummy with an honor, however, you will win the ace and watch what West plays. If he follows suit, you can clear diamonds by cashing the king and exiting with another diamond. If West shows out on the first round, however, you can just play a low diamond back toward dummy’s 8. This play allows you to overcome the layout at right where East started with four diamonds. East can win the ♦8 with an honor, but you will win whatever the defense returns and take the marked diamond finesse to score four tricks in the suit.