You are looking for unprotected enemy pieces and for checks, hoping to link them for profit. Scan for unprotected White pieces and you find the knight (as well as the queen, but again the knight makes a more feasible target for a fork). Now look for checks, and in particular for a match—a check that also attacks the loose knight. Qf5+ or Qf1+ work (it’s important to see both, since they may have different side consequences), attacking both knight and king, and winning the former after the latter moves.