...then probe the medical history or run blood tests before calling it a duck?
I'm sure that we've all encountered this, either directed at ourselves or at others, but there seems to be no shortage of people out there who won't accept the gender identities presented by trans people (transsexuals in particular). They consider us to be no different than cross-dressers, ->-bleeped-<-s, and drag performers (to whom I refer with all due respect), no matter how hard we work at our transitions to be passable. They might say things like, "once a man, always a man."
Ironically, most of these people are the ones who do the most gay-bashing and who like to tease or smear non-gender-conforming cis people by calling them gay (or some homophobic slur). Also, men who are not considered "manly" enough are often insulted by being labeled as female. They want to have it both ways, that gender is a matter of performance, but only reserved for those with the "right" genitalia. What bothers me is that I don't see people calling them on this. Although it might not help, there's more to gain than to lose if they are challenged to provide coherent and consistent definitions for what they think that "men" and "women" are supposed to be.
It's not just ignorant and thoughtless people (if they're not outright bigots) who do this. I have seen educated and rational-minded people (who pride themselves as such) fight tooth and nail to insist that a trans woman is not a "real" woman (or that a trans man is not a "real" man) because the chromosomes or the shape of certain bones (i.e., things that can't be seen without without medical testing) are not the same as what GGs have. And they persist their arguments even when presented with evidence showing a neurological basis for ->-bleeped-<- or examples of intersex conditions.
There's so much transphobia and cisgenderism/cissexism in the world that people can't or won't recognize unless a hate crime is involved, and it always upsets me just as much as the hate crimes do. The former enables the latter, in my opinion.