Yesterday, I came across a bird on a car in a parking lot. The food it had in its beak made it look, from the front, like a duck. I am not sure, however, whether ducks have spots, so I can't determine if it looks like a duck. I am not sure how ducks are supposed to walk--nor, for that sake, how other birds walk--so I cannot use that criterion. The bird did not quack, but one cannot infer non-duck status based on the converse. Another practical problem is that I am not sure if there are any "non-duck-like" ways of quacking (other than in offering bogus medical services), so I am not sure what the standard of comparison would be.
Ultimately, of course, the question is whether I could make more profit on this bird than I could on a duck. Unfortunately, I did not see any commercial value here.