White has a loose bishop at a4 (and also two loose rooks which are less accessible). Is there a square from which Black's queen would be aimed at both the bishop and White's king? Hmm: h4 is an interesting start, as from there the queen could give check and would win the bishop if the e4 pawn weren’t in the way—always assuming Black's queen could get to h4 past the knight at f6. So Black begins playing with ways to move both the knight and the e4 pawn out of the way. The natural method is to use the knight to attack something the e4 pawn protects—like the d5 pawn. 1. … Nxd5; and then if White replies e4xN, 2. ...Qh4+ wins the bishop.
You might just as well have seen this by playing with any captures you can make and asking what would be possible on the board as it would look afterwards. This leads you to Nxd5; after imagining the recapture e4xN, you look for any new checks you would be able to give and see the fork Qh4+.