Gather in the Hall of the Planets — Barry N. Malzberg

Science fiction writer Sanford Kvass has a problem. Three problems, actually. He suffering from terrible writer's block and owes his agent a large sum of money. The last thing he needs is the approaching distraction of the World Science Fiction Convention, with it's obsessive fans, sex-mad SF groupies and professional writers and editors getting drunk and behaving badly.

But we said 'three problems', didn't we? The best that can be said about Sanford Kvass' third problem is that it renders his first two irrelevant. Kvass is approached by an alien ( a genuine alien, not a cosplay one) who informs him that the human race is to be tested: an alien will appear at the World Science Fiction Convention, disguised as a human being, and unless Kvass can unmask it, the Earth will be destroyed.

Under normal circumstances, this wouldn't present much of a challenge. All he's have to do, is to observe as many people as he could and identify the one who clearly had no experience of normal social interaction. Voila! One unmasked alien.

There's just one problem: this is Worldcon . . .