(Spoilers Extended) If Aegon is indeed a fake, how would it even be revealed?

Jon Connington sincerely believes that he's real. Aegon believes himself to be real. Illyrio and Varys have no incentive to admit that he's fake if he is, and neither of them are likely to be POV characters. And it's not as if he can get a paternity test.

If he is a fake, how would it even be revealed to us, the reader?