Just before Fredo's boathouse outburst, when Michael, Tom and Neri are discussing how Pentangeli is alive, I reckon Tom says gleefully! payback time!!

“No way we can get to him. If we don't it's up to five counts of perjury”
"Fredo says he doesn't know anything I believe him"
“Roth played this one beautifully”

I reckon he is thinking I am gonna take over! I am gonna be the Don!!