First, does White have any loose pieces? Yes, the bishop at d3 (and also the rook at a1, but it’s inaccessible for now). You would like to check White’s king and attack the bishop at the same time, so look for a square from which the queen might do it. You see that e3 fits the bill, and that the queen could reach it directly if Black’s own pawn at e5 weren’t in the way. So ask whether the e5 pawn can vacate its square in a hostile manner that requires a time-consuming reply. Yes: 1. …e5xd4; 2. exd4 (or cxd4), Qe3+ wins the bishop. You likewise might have seen this by imagining the consequences of pawn trades available to you in the center. You see that the first round of captures just described leaves the queen with a clear path to a new check—and fork—at e3.
The likely payoff from seeing all this, of course, is the gain of just a pawn; for if your opponent sees the fork coming—and you should assume he will—he will not recapture after exd4.
By the way, it also might have occurred to you to start with e5xf4 (instead of taking d4); this likewise moves Black's pawn out of the way. What's wrong with it? The trouble is that this time White can reply Nxf4, and then his knight suddenly protects the bishop on d3 that you had counted on as a target: it's no longer loose!