Look for visual patterns; observe that the king and rook are on the same diagonal with nothing between them. This cries out for a double attack by a bishop (or a queen acting like a bishop); here, that means Bc4+. Or you might find the answer with a routine look at forcing moves. What checks can White give with his bishops? Two: Be8+ (resulting in KxB—not very appealing), and Bc4+, Bc4+ also attacks the rook at a2—which White wins. Yet another way to have seen this would be to ask which Black pieces are loose, which leads to the rook; the only way White can attack it is with Bc4, which also checks the king.