Definitely at the meeting with Mike, Clemenza, Vito, etc. is when Tessio realized that it was over. He obviously disagreed with the strategy put forth by Vito & Mike:

TESSIO'S VOICE (OS)
I'm not asking you for help, Mike -- just take off the handcuffs.

MICHAEL'S VOICE (OS)
Be patient.

CLEMENZA'S VOICE (OS)
We got to protect ourselves -- Give me a chance to recruit some new men.

MICHAEL'S VOICE (OS)
No -- I don't want to give Barzini an excuse to start fighting.

TESSIO'S VOICE (OS)
Mike, you're wrong...



One question I have was whether Clemenza & Tessio knew that Vito & Mike were setting up Carlo. If Tessio & Clemenza did not know, then Mike's appointment of Carlo as Michael's "right hand man in Vegas" was probably another confirmation to Tessio that Mike would run the Corleone Family into the ground.