How did Galadriel knew that Gandalf was back?

[Mungo of Bracegirdle]

Also, wouldn't Sauroman and Sauron be able to communicate without the Palantiri?

Good point, possible answer:

Saruman could hide his deep thoughts well. Examples, none of the Council preceived his real intentions (finding the ring), for a while he even fooled Sauron of his real intentions, Radagast is fooled by him, as is Gandalf. That is one of his most potent powers, to hide his feelings, plans, intentions. He truely is a wolf in sheep's clothing. So there seems to be some sort of control over how much of your mind someone can preceive. The palantir was only an instrument by which Saruman was ensnared. My thinking is that he used the palantir exclusively to "talk" to Sauron, but kept the rest of his mind hidden. Remember too that Sauron had put much of his power into the ring. So perhaps his ability to see the thoughts of others was lessend. Somewhere (I don't remember where exactly) it says that with the ring Sauron could "see" right into your very soul. Because he does not have the ring, we can assume (I think) that that power is limited as the entirity of it lies within the ring.
Hope that was clear.

Cheers,
Mungo