Within minutes of the Tahoe attack, Michael was almost certain that Roth was behind the attempt. I believe he saw that Roth had set it up so that Frankie would look like the patsy. Frankie came to the party sore at Michael for supporting Roth and the Rosatos against Frankie. Therefore, Frankie had the "perfect" motivation to whack Michael--and would make the perfect fall guy.
But Michael saw that the plan was too obvious, too pat. Frankie wasn't smart enough to plan the hit, much less smart enough to take over the Corleone empire. So Michael put his "Sicilian cunning" to work. Michael had been horning in on Roth's Western gaming empire since he returned from Sicily. He realized that Roth had been stringing him along by pretending that Michael was his heir-apparent; by doling out little pieces of his empire (like Klingman's share of the Tropigala), and by promising Michael his Cuban interests. Now Michael saw that Roth, not Frankie, had the most to gain from whacking Michael: rid himself of Michael's constant encroachments, take over Michael's Nevada gaming interests, and, by making Frankie the fall guy, turning him into a hunted man--so that the Rosatos, who were loyal to Roth, could take over the Corleone interests in NYC.
I said that Michael was almost certain it was Roth. The clincher: when he visited Frankie, he made it look like he blamed Frankie--and Frankie was turned into jelly by Michael's anger. But when Michael said he wanted Frankie to help him "take my revenge" on Roth, Frankie practically passed out with relief. Sure enough, Frankie said, "Michael, anything..." That's when Frankie passed the test. Then Michael, in one of his most brilliant manipulative moves, said, "Settle these problems with the Rosato brothers." In effect, he told Frankie to take a mortal risk that he wasn't willing to take before Michael's visit. Now he couldn't lose: If Frankie did settle his problems with the Rosatos, it'd be one less worry for Michael. If Roth had the Rosatos whack Frankie, it'd be one more proof that Roth was behind the hit.
This is a very long way of saying that Michael at no time suspected Frankie in the Tahoe attempt.