I'm just watching "The Search" where a treaty is apparently being signed with the Dominion.
What was the first clue that things weren't right?
My supposition: when 'Nechayev' called Sisko 'Benjamin'.
Because she knows Picard better, but she'd never call him 'Jean-Luc'! Of course, Sisko wasn't about to tell an Admiral how to address him (!) but this was still something Nechayev wouldn't do.
What else could have been a clue?