@Great Argus - the most obvious differences are leg colour (Pied Shags don't have yellow legs/feet), the black plumage on the leg/vent area (in Pied Shags it forms "trouser legs" while on Spotted Shags the whole vent-rump area is black like they've been dipped in black paint), and Spotted Shags have spots on the back. The Spotted Shag on the whole is a more gracile-looking bird with a finer bill so if you're used to them you can pretty much tell the species apart at a glance, but otherwise you only need to look at the back half of the body (legs, vent, spotted back).
