At the end of "The Survivors," Captain Picard tells the Douwd's wife that she did, in fact, die, while attempting to defend the colony against the Husnock. How could Picard know this? I know of nothing in the episode which showed this, and why did he assume that Kevin wasn't human, and not Rishon (or both)?
IIRC: Captain Picard suspected that the alien ship was not quite what it seemed when he noticed several inconsistencies:
1.) Given its tremendous capacity to be impervious to the Enterprises weapons and still be able to disable all of the Enterprise's weapons with a single blast; the ship could have easily destroyed the Enterprise. When the Enterprise first escaped (miraculously, btw, as no crew were harmed, nor any systems badly damaged)
This helped Picard establish that the goal of the Husnock ship that the Enterprise encountered was to chase them away, rather than destroy them.
2.) Kevin Uxbridge, himself, told the Enterprise that the Husnock were the race responsible for destroying the entire population of the planet.
This seems to be at odds with the goals of the ship that the Enterpise just encountered. This discrepancy was further cemented when in the final encounter, Picard declared that they would not go anywhere with Federation citizens still on the planet. So the alien ship immediately proceeded to destroy the villa. Exactly what would be required for the Enterprise to leave them alone.
3.) Kevin Uxbridge seemed to be the most vocal about having the Enterprise just "leave them alone". Several times in the episode, Kevin made statements and claims that would be seemingly at odds with what would be a logical move to secure the couples' (Kevin and Rishon's) survival. I.e., leave with the Enterprise, etc.
This tipped off Captain Picard that the Husnock ship was either: in contact and working with the Uxbridges, or that he was controlling it. I do not believe it was actually established until Captain Picard beamed the pair on board.
As for the rest of the question; I believe that Kevin was the one to confess to Picard about his Douwd origins.
NOTE: It has been a few years since I've seen this episode though. So if Picard actually accused Kevin of being an alien (not the same as "in league with" the aliens) I would appreciate knowing when that happened before Kevin actually revealed himself.