Whether you look for Black pieces on common diagonals or ask whether White has a way to attack two pieces at once, the same answer appears: Bd7. The move seems pointless because Black just plays QxB. But ask what checks White then would have. There are four: Qe8, Qe7, and Qb4, none of which work; and QxR, which is new (well, the move itself isn't new, but it didn't give check in the initial position), and which leads to checkmate a move later. (If Black blocks the check with Qe8, White has QxQ# with protection from the rook on e1.) Since Black can’t afford that, the initial fork Bd7 actually works well, picking up the knight.
The key to seeing this, as ever, is to be thorough in examining checks and their consequences—not only on the board in front of you but on the board as it would look after whatever forcing moves you can imagine. You likewise want to be alert to how any recaptures you can force would open lines or leave things loose. Here the Black queen’s movement off the back rank opens a line to the king from a8, and turns out to be fatal.
It is good to see the bishop fork in this position, but there is another route to a similar result that is worth seeing as well. All sorts of possibilities spring up once you realize that Black’s queen needs to stay on the back rank to prevent White from mating with QxR; for this means the Black queen itself is vulnerable to attack. White therefore can play Bxa6, putting Black in a pickle. If he plays QxB, he promptly gets mated; so he moves his queen to d8, where it is safe and continues to protect the rook on a8—but now he has left his knight loose, so White plays QxN. Black’s queen was overworked, a theme we have seen before and will study in detail later. There is more than one way to take advantage of such a situation, as this analysis shows.