to be convinced · to be sure · to firmly believe · to be positive that · definite news

I'm pretty sure I can help him.

You seemed convinced that Tom needed to do that.

Person of the future: "The calendar of this Unix machine that the ancients built only goes up to January 19, 2038. As such, I am convinced that this marks the day of the Earth's destruction."

Do you know for sure that Tom is behind that?

We'll find a solution, I'm sure.