What checks can Black’s bishop inflict? None, of course, not least because the bishop is on a light square and the king is on a dark one. But the king is just the best anchor for a fork, not the only one; you really are looking for a square where your bishop might be able to attack any two vulnerabilities at the same time. Here the bishop can move to c6 and attack White’s b7 knight and h1 rook, both of which are on the same light-squared diagonal and both of which are loose. Black wins the knight.
If you step back from the board and look for visual patterns, the alignment of White's knight and rook with nothing between them should be conspicuous. Use these studies to gain the habit of spotting enemy pieces like these that are on the same line (not to mention loose).