If the glue is for Plastic and Pewter, chances are good you've got super glue of some kind. [Cyanoacrylate] As mentioned above, nail polish remover contains acetone, that breaks down cyanoacrylate.
Unless the parts are very fragile, you could probably crack the pieces apart with a bit of pressure. Drop a model on a concrete floor, and you'll get two neatly separated pieces in the least convenient location.
If there are ball joints, you might have a problem, but working a knife into the join to loosen things up would probably help. It would also help to let the acetone get deeper into the join.
Go easy, and keep the pointy sharp bits away from yourself. The hash marks of old scars still disfigure my finger prints, which makes it super easy to identify me at a crime scene. There's a reason I always wear neoprene gloves. Not just for fetishists, they're also useful for creating reasonable doubt!