The Phantom Menace has been revealed, so the Jedi know that the Sith are back. Some of the Jedi, such as Yoda, can sense the Dark Side clouding the future, making everything uncertain. And then there was the incident on with the Sith warrior that ambushed Master Qui-Gon on Tatooine. The one that was later defeated on Naboo. There have been other sightings as well, but nothing as substantial. There are also rumors that link the Sith to the Confederacy - or even claim that the Sith have infiltrated the upper echelons of the Republic! But again, nothing substantial.