All forks that are applied with knights and bishops have one thing in common: The pieces being forked must be on the same color. The first step, then is to look for pieces that are on the same color.
For bishops, the pieces must be on the same diagonal, where you can put your bishop between them. Or, if the pieces are on two different diagonals and there is another enemy piece where the diagonals intersect, such as this king-rook fork.
Next, learn to recognize the positions from which a knight can fork two pieces, as illustrated below. Also note that, with the exception of the windmill, these forks can be applied by the knight being in either of two places.
