I think it has all but been given away openly.
Buffy has a prophetic dream sequence where Xander is a bot.
Buffy is placed under a spell where only someone devoted and in love with her can awaken her. The next panel shows Xander. If Xander is a bot, his kiss would be ineffective as he isn't a real person.
Willow is the only person aside from Warren who has knowledge of how a bot is created and maintained.
Willow and Twilight can both fly.
Special care is taken to mention Xander's missing year with Dracula.
Dracula has knowledge of gypsy tricks and magic.
Special care is taken to mention Willow's missing 6 months after Kennedy dies and is resurrected and her fears of losing her again.
Special care is taken to mention Willow's siding with Buffy and the cost to her (Tara) and how that will not happen again.
Special care is taken to mention that a witch was present when Dracula "lost" his secrets in a card game.
Special care is taken to have Willow quote that she would not betray Buffy
anymore than she already has.
Twilight has intimate knowledge of Buffy and how to break her.
The Japanese vampires have knowledge of not only how the slayer brigade was created, but how to reverse the process.
Giles has now recruited Faith to handle rogue slayers, which in the end will pit Faith vs Buffy for a final showdown - pitting slayer vs slayer with no ultimate winner since the victor will also be eliminated.
Twilight is the real Xander who has been trained by Willow to be a Warlock.
Xander, Willow and Giles are working in concert together to remove the demons, slayers and magic altogether from the world - for the good of the world. Interestingly, these 3 are also the ones who are directly tied to Buffy with the spell cast to infuse her with the essence of the 1st slayer.
All 3 are betraying Buffy and working with the government, leaving Buffy literally against the world.
For Xander, betrayal of Buffy is no new thing as he has done so repeatedly throughout the previous 7 years - albeit mostly from jealousy of her boyfriends.