Figure[Black to move]

First observation: White’s rook is unprotected. (So is his queen, but it’s a harder target to go after.) Second observation: in Qd4, Black’s queen has a safe check that attacks White's rook.

So there is the idea of the fork. But then you also want to make sure White can't wriggle out of it. His natural idea would be to try to move the rook away and block the check all in one move with Rf2. So suppose he does; how do you figure out whether this reply spoils your idea? Ask the cardinal question: consider what your next check would be, and with what consequences. Here it's Ra1, which forces White's king to h2. His rook has been left loose, so Black takes it: QxR.