Most likely it's on a morse taper shaft. Get a puller- either there are two or 3 threaded holes in the face of the adapter that bolts will thread into or you'll have to use a 3-jaw and reach arount to the back corner to pull it off. I've seen guys use an air chisel with a punch bit and hit the center of the shaft while someone else pulls out on the wheel but I'd be concerned with breaking something. Sears and Harbor Freight could probably fix you up today if you don't own any of these tools. Auto Zone has a free loaner program too.