from the Vice article:
I asked jokes_on_you how they spotted the difference. "I’ve sucked enough toes in my life to recognize when something doesn’t look right," they told me in a Reddit direct message. "Because we can’t dorsi- or plantarflex our 2nd-5th toes independently I knew it wasn’t a matter of the toe being bent. I thought that maybe she has some form of brachydactyly but her Wikifeet page has clear evidence to the contrary. So it was clear to me that it wasn’t her feet."
(the comments are great)