Using a 1X4 (1X2, 1X3, etc), set it so that it pivots from the trailers hitch and swing it 180* from the left side of the trailer to the right side of the trailer. Use a sharpie marker, or ?, and mark the point where the trailer protrudes the farthest out on the stick, then measure that distance and add a few inches for clearance. This will be your beginning point of measurement needed for clearance from the cab of your truck.
Don