This is what Michael tells Tom before he heads off to Miami.
He never tells Rocco or Neri, but it appears they fell into line and took Orders from Tom (i.e. Neri's "help" in setting up Geary). Michael seems to know instantly that it was Roth who tried to kill him, because he goes directly to Roth after the attempt, to "keep his enemies closer." But did he tell Tom he suspected Roth? Not in the script he didn't, and that's a huge mistake because if Tom had known it was Roth maybe he would have paid closer attention to the "death" of Pentangeli, through his sources in the NYPD. This would have allowed Don Tom to do something either to get word to Pentangeli to keep his mouth shut, or to start to prepare for the inevitable hearings that Pentangeli's confession would cause. Once again, Michael's secretiveness is a big part of his undoing. Moreover why did he have to go to the trouble of telling Tom he was going to be "the Don." He never really was anything more than an acting Don while Michael went to Miami, New York and Havana.