Start by examining every check White can give. There are four: Ng6, Nf7, Qxh7, and Rf8 (don't forget to consider every attacker). The knight checks are important because they show you that Black’s king can be attacked with the knight in just one move; they suggest that White might have a crushing fork if Black’s queen could be forced onto a dark square. Qxh7+ just loses the queen and doesn’t help the forking prospects. Rf8+, however, permits only one reply: QxR. Visualize the resulting position and see that it calls for a knight fork at g6. Ask whether the square is protected and see that the h7 pawn appears to be on the job—but it’s pinned. White wins a queen for a rook and a knight.