There isn’t an official version of Poe’s Law, but basically it is:
A parody of an extremist will be indistinguishable from the real thing.
In a sense this is all but definitionally true, since parody is making fun of something by presenting a more extreme version of it. If something is already maximally extreme, there is nowhere to go with a parody, so a parody will consist of saying the same things.
But… this is not quite true. It is possible to distinguish between an extremist and a parody because the extremist has a different goal than the parodist does. The parodist seeks to make people laugh. The extremist is trying to live life, and no matter who you are, life is primarily mundane. If you pay attention to what an extremist says, you will notice that most of what they say is actually fairly boring.
This stems from something Chesterton observed: a madman seems normal to himself. Since he is normal, he doesn’t think about his extreme views differently from his normal views, because to him none of them are weird. It’s not that’s he’s unaware that most people disagree with his extreme views, but that the disagreement is what will be weird to him, not his own views. We think of his extreme views as some oddity tacked on to the rest of his normal views (such as eating when hungry, sleeping when tired, and washing his hands after using the bathroom). He thinks of his extreme views as fitting in with the rest, since there’s on reality an so everything that’s true about it necessarily fits together. The result is that when he speaks, much of what he says will be prosaic, because he has no reason to speak only about his extreme views. People like to talk about the world, not merely the occasional isolated belief about it.
We thus have a way to tell the difference between an extremist and a parody: the density of extremism in the expression. Or, to put it another way, how funny the thing is. The true extremist isn’t in on the joke, so he doesn’t take care to only talk about the funny stuff. The funny stuff may not even interest the extremist all that much. The parodist, by contrast, is in on the joke, so he takes care to avoid the boring things a real extremist would say.
To put it succinctly, brevity is the soul of wit and the parodist can put on the extremist’s clothes, wear a wig, and even use makeup to change the color of his skin, but he can never change his soul.