Top
Start at the topmost corner of the inclined plane.
Drop
Drop a vertical line from the topmost corner and make a right triangle with the inclined plane as the hypotenuse.
Sweep the Clock
At the top corner of your right triangle, sweep the angle from the vertical line to the hypotenuse. Start at o'clock and turn the angle to the inclined plane.
Sign of
If the angle sweeps in a counterclockwise direction, is positive. If the angle turns in a clockwise direction, is negative.