Looks like a duck, swims like a duck etc.

There are many forms and versions of this idiomatic phrasing that describes a simple form of reasoning that has come to be known as 'The Duck Test'. One of the most common forms is, 'If it looks like a duck, swims like a duck, quacks like a duck, then it probably is a duck.' It's just another way of stating the obvious, or stating something or other that is generally accepted. Some sources attribute the origin to the French inventor, Jacques de Vaucanson, who built a mechanical duck in 1738, but it is highly unlikely that he originated the idiomatic phrasing. It would appear that the earliest citation is from Emil Mazey, secretary-treasurer of the US United Auto Workers Union, who at a union meeting in 1946 accused a person of being a Communist, 'I can't prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks—I'm certainly going to assume that he is a duck'. Mazey's words were widely publicised in the press, and it seems 'The Duck Test' had been born.