It should be spot welded. Unless it is not original and has been replaced before and was installed with panel adhesive. If it's original, it should be spot welded. Look on the lip of the inner wheelhouse for the spot welds. You can also take a small sanding block and sand around the lip lightly. THis should exposethe spot weld reliefs. Or just start splitting the seam with a small chisel to see where it's attached. Or if you're good with a hammer and dolly, just air chisel it off...but that's more destructive. If you don't want to mess with it at all, you can cut the wheel house off and leave the lip attached, and weld the new one right over it. Not what i perfer, but have seen it done. The amount of thickness increase is not significant.