Good question!
Logically, Fredo would tell Roth, since he'd seemingly cast his lot with him. But, logically, if Michael couldn't completely trust Fredo, why would he tell Fredo about his plan to whack Roth? And if Fredo had told Roth, why didn't Roth accelerate his plan to whack Michael?
One thing seems clear: By the time of Michael's last meeting with Roth ("Who had Pentangeli killed?"), both of them seemed to know each other's plans. What makes the scenario convincing is how both of them were hanging tough at that point: Roth holding out for the $2 million, Michael holding out for finding out who the traitor was in his family.