@thomasfish
Asuka knew immediately, more or less. Mayu wouldn't have noticed because she didn't see men at all, she only saw monsters. However, I reckon she would have noticed very soon even if Kei hadn't come out of the crossdress in front of her. Considering she can now even talk to Kei in his male form as well, she shouldn't anymore be seeing him as a mass of threat. In fact I thought she might already suspect it, but I guess not, based on this page.
Asuka was funny in this strip, despite saying nothing. First she looked worried, then all "Ehhh...".