You want to spot the kernel of a discovery every time it exists. Here White’s knight masks his queen (don’t be distracted by the queen's backward path of attack). Black’s queen is loose, and so makes a perfect target; White’s knight will give check, and then White will play QxQ.

But which check should the knight give: Nd5 or Ng8? When there are two checking options, as there often are with a knight, carefully consider them both. Start by asking how Black would reply to Ng8. His only capture would be Ra8xN. Visualize that move and notice that it pins White’s queen to his king (the alignment of White’s king and queen is a conspicuous feature of the position). White then can play QxR, but not QxQ—and now Black has QxRa1+. So Ng8+ doesn’t work. Now consider Nd5+. It leads to RxN or e6xN, either of which is okay with White; he plays QxQ regardless, ending up with a material advantage after starting the position behind by one rook and threatened with the loss of another.