This has always been a confusing part of the story. I'm thinking Michael thought Pentangeli was killed in that bar because Willie Cicci survived and assumed Frankie never got out of that bar alive and told Michael that. Frankie was then hustled away out of sight until he was to testify. It's confusing because it would be hard to almost kill Frankie without Frankie waking up and wondering how the hell did he survive a hit from Michael Corleone?

I'm with the camp who believes that Roth had ordered that Pentangeli be
almost
murdered so that he would turn on Michael. When Michael asks who had Pentangeli killed, Roth gets this look on his face like, "Oh, yeah, I'm supposed to pretend that Frankie is dead". He then becomes a bit uncharacteristically disheveled as he tries to pawn it off on the Rizatto brothers (a ridiculous answer considering the Rizzato's were hardly in a position to "have" someone killed). If Roth truly believed that Frankie was dead, he would have launched right away into the histrionic Moe Green story when Michael confronted him.
He who conquers himself is mightier than he who conquers a city.

No one knew Frank survived the attempt until he was to testify the following Monday

Shouldn't Michael ask who attempted to kill him?
Maybe everyone thought Frankie was dead, but was secretly put in witness protection when he survived.

My theory: Michael already KNEW Roth gave the order to kill Pentangeli - or, more accurately, to NEARLY kill Pentangeli, and make Frank think Michael had ordered it. Michael was also banking on Roth not knowing that Pentangeli had survived, so Roth would think that Pentangeli hadn't been turned against Michael.
If anybody BUT Roth had ordered the hit, Michael knew Roth would tell him who it was. When Roth deflected the question, Michael knew Roth was against him.
That's how I saw it.

• Crazy. All crazy but I'm.