White’s knight is on a light square, along with Black’s king and queen. Examine every square the knight can move to and a fork at d8 suggests itself; but d8 is protected by one of Black’s rooks. You look for ways to pin, capture, or distract the rook, and find none. But before giving up on the thought, imagine carrying out the fork and letting the knight get taken; ask how the board would look with the knight gone and the rook moved to d8. Methodically imagine any checks you then would have. The most interesting is Qxe7, since it's safe and attacks not only Black's king but also the now-loose rook on d8. How would Black respond to the latter threat? With Kg8; it would be his only legal move. Now White has the simple QxR+. Here the fork is just a means to an end, viz., capturing the rook and winning the exchange.