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The Duck test is a humorous term for a form of abductive reasoning. This is its usual expression:
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If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.
The test implies that a person can identify an unknown subject by observing that subject's habitual characteristics. It is sometimes used to counter abstruse, or even valid, arguments that something is not what it appears to be.
History
Indiana poet James Whitcomb Riley (1849–1916) may have coined the phrase when he wrote:
When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.
The more common wording of the phrase may have originated much later with Emil Mazey, secretary-treasurer of the United Auto Workers, at a labor meeting in 1946 accusing 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.
The term was later popularized in the United States by Richard Cunningham Patterson Jr., United States ambassador to Guatemala during the Cold War in 1950, who used the phrase when he accused the Jacobo Arbenz Guzmán government of being Communist. Patterson explained his reasoning as follows:
Suppose you see a bird walking around in a farm yard. This bird has no label that says 'duck'. But the bird certainly looks like a duck. Also, he goes to the pond and you notice that he swims like a duck. Then he opens his beak and quacks like a duck. Well, by this time you have probably reached the conclusion that the bird is a duck, whether he's wearing a label or not.
Later references to the duck test include Cardinal Richard Cushing's, who used the phrase in 1964 in reference to Fidel Castro.
Douglas Adams parodied this test in his book Dirk Gently's Holistic Detective Agency:
If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family Anatidae on our hands.
A claimed recent application of the duck test was the denial of tax exempt "nonprofit" status to Blue Shield of California.
The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test:
If it looks like a duck and quacks like a duck but it needs batteries, you probably have the wrong abstraction.
Russian Minister of Foreign Affairs Sergey Lavrov used a version of the Duck Test in 2015 in response to allegations that Russian airstrikes in Syria were not targeting terrorist groups, primarily ISIS, but rather West-supported groups such as the Free Syrian Army. When asked to elaborate his definition of 'terrorist groups', he replied:
If it looks like a terrorist, if it acts like a terrorist, if it walks like a terrorist, if it fights like a terrorist, it's a terrorist, right?
Elephant test
Similarly, the term elephant test refers to situations in which an idea or thing, "is hard to describe, but instantly recognizable when spotted".
The term is often used in legal cases when there is an issue which may be open to interpretation, such as in the case of Cadogan Estates Ltd v Morris, when Lord Justice Stuart-Smith referred to "the well known elephant test. It is difficult to describe, but you know it when you see it".
A similar incantation (used however as a rule of exclusion) was invoked by the concurring opinion of Justice Potter Stewart in Jacobellis v. Ohio, 378 U.S. 184 (1964), an obscenity case. He stated that the Constitution protected all obscenity except "hard-core pornography." Stewart opined, "I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that."