The only person Mike completely trust was Tom. Everyone else he feeds misinformation to see what comes of what. He doesn't know when he going to kill Roth until what Fredo says at the show. So telling Fredo before hand is purely misinformation.

What I don't understand is how he knew there was a traitor to begin with? Hundreeds of people were just at your house, none of them could find your bedroom window? Why not suspect the senator? Maybe him and Turnbull had hatched the plan lol.