American Crow (Corvus brachyrhynchos) or a Fish Crow (Corvus ossifragus)?

The question, inspired by creeping one my favourite blogs: Ontario Birds and Herps which describes a recent rare sighting, is answered in an interesting article: http://www.birds.cornell.edu/crows/FishCrow.htm

The short answer, according to the article: Ask the bird if it's an American Crow.  If it's a Fish Crow it will emphatically proclaim "uh uh!"