White’s knight is on a light square, as are Black’s queen and knight. The structure of a fork is there; if the Black knight could be replaced with the king, the fork would become playable with Nc5. So ask whether the knight can be taken by any of White’s pieces, and see that one of them—the rook—is ready to do the job, and with check. After RxN+ Black appears to have a choice of recapturing with his queen or king, but on closer inspection the queen is pinned. So the only recapture is KxR; and then Nc5+ follows. And again the same result can be reached by just experimenting from the outset with the check RxN+. After the recapture KxR you see Black's king and queen in the telltale position for a knight fork.