Remove the blade, if necessary, by holding the blade so it will not turn, and removing the center lock nut with a wrench.
Identify the top and bottom of both wings of the blade by finding the side that has the sharpened cutting edge. The blade will taper up from a thin edge into the body of the blade on the bottom side. The top side will not have a taper but be of an even thickness. The cutting edge may have what will appear like a strip on the edge that is a different color than the rest of the blade. The difference in color comes from the sheen of the sharpened metal.
Identify the top and bottom of the sharpened edge. The bottom of the sharpened area is the edge of the bottom of the blade. The top is the width of the area sharpened, usually an area that is evenly 1/4-inch wide when measured from the edge.