If Thor wasn't sure of it when Tony nearly plowed into him and he got a whiff of the alcohol hanging off him, he was certain of it from the casual tone of Tony's voice:

Tony Stark was drunk.

He doesn't bother asking him if he's been drinking, because he absolutely knew that Tony had been.

"Things are fine, Loki is fine. Not a kitten, though." And sadly, not a snake either.

"How are you?"