Bingowings said:

He didn't presume it was the other Jedi who seemed certain of it.

No Yoda knows with certainty about the rule of two. "Only two there are. No more no less."

Yet how does he even know this if the sith were extinct or even hiding for a millenium?

Did he get an anonymous email from a "thard eban"?.

I mean really how the hell does he figure it out?

edit: bingo, that expository scene in the temple would've been so much better. Actually, you could also do it with Jake's "excuse me Qui-Gon what are midis" speech too.

Imagine your temple master played by the one and only Chris Lee as Dooku.