Figure[White to move]

Black has one loose piece: the rook at h7. White has one check with the queen: Qg8, which also attacks the rook. It all looks fine except that Black guards g8 with his knight. So press forward with the next question: Does White have any pieces attacking the knight? Yes, the bishop at c5 and the knight at f5. (The two attackers are important; do you see why?) Thus 1. BxN (or NxN), and after Black's recapture, g8 is available for the White queen to administer the fork.

The answer to the question about the two attackers is that the double attack would be foiled if Black could recapture on e7 with his king; for then White no longer would be able to give check with Qg8. But Black's king can't make the recapture because whichever piece White uses to capture on that square is still protected by the piece he didn't use.