If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.
Popularized by 1950 United States ambassador to Guatemala, Richard Cunningham Patterson Jr., accusing Guatemala's goverment of being a communist one.
The duck test is a humorous description on how to intepret vivid signals when the rest of the world insist in something totally different.
Why everybody else might see something different? It is a matter of perceptions, thus a matter of subjectivity: it will vary from person to person.
This subjectivity is the key to apply the duck test for our everyday life. It will just work in our own realm of influence, but not beyond. Even if we perceive something as obvious, reasonable or convenient, we cannot force others to acquire this vision; we will only be capable of influencing those under our control.
Other parties might simply not see the duck. Or even may be more convenient for them that thing not to be a duck. Here another aphorism comes into play: the one that goes by markets can remain irrational longer than you can remain solvent. The point is that one cannot fight alone agaist everybody else.
Bringing this to the IT realm: if something during a project presents a clear face, use that knowledge for you decisions even if other members in the project reject this interpretation. But the key is to not try to impose this vision to others; it is even more important when there is hostility towards that vision (ignoring it is a clear sign of hostility).