In order to be "discovered" to be an AI, it must not have been publically known prior to the discovery.

What if it posts something like sports scores or stock prices, and nobody knows whether it's an AI but nobody really cares?

@StevenK Hmm, good question. I won't count that. I'm thinking of situations where people had an assumption it was human, and may be upset to discover it was not.

@IsaacKing What if an account has been human-written for most of its history, but at some point reveals that an AI has been writing its tweets for the past month? week? day?

@StevenK The AI needs to have been a major contributor to the account's popularity. Past day and week, probably not. Past month, maybe, if the AI posts were getting as many or more likes as the human posts before it.