Inspect for checks Black can give with his bishop; this turns up Bd4. Does the move attack anything else? Yes, White's queen at b6. Since the queen can attack the bishop, the fork only works if the bishop has protection on d4. It does, from the Black rook on d8. So Bd4 trades the bishop for White’s queen. Now try seeing the pattern by scanning the geometry of White's pieces; it is important in a position like this to notice right away that White's king and queen are on the same diagonal. Again, it will help to take away two types of lessons from these positions: conceptual points (trains of thought that lead to the forks) and visual patterns that you will be able to recognize in your games.