[Boromir88]

I agree with Lal that Sauron must of had a very short list of possible candidates for the Elven Ring bearers. It wouldn't surprise me if he guessed that Gil-galad had at least one of them then figured out through some logical thinking that Elrond...being Gil-galad's herald and named heir would have received a Ring. Which would leave of the Elves likely candidates being Galadriel, Cirdan, and perhaps Glorfindel. I also agree that it would probably be hard for him to figure out that Gandalf had one of the Elven Rings, I don't see Sauron really getting any info about Gandalf having one of the Ring. As his was passed down to him by Cirdan. Anyway, that's just all guessing as far as to whom Sauron thought may of had the Elven Rings.

As far as Saruman, I think it's more likely who knew who held the Elven Rings. They were his 'specialty,' he already came to guess that Gandalf had one of them:

And the Grey Messenger took the Ring, and kept it ever secret; yet the White Messenger (who was skilled to uncover all secrets) after a time became aware of this gift, and begrudged it, and it was the beginning of the hidden ill-will that he bore to the Grey, which afterwards became manifest.

Saruman, the 'skilled uncoverer of secrets' figured out Gandalf had one of the Elven Rings...it wouldn't surprise me as head of the White Council (and being members themselves) he would have figured out Galadriel and Elrond also had 'em.

nethertheless didn't saruman know so why didn't he tell sauron

I think Saruman did know about them all. However, that's not sure...it is rather certain that he knew Gandalf had one of the Elven Rings. Anyway, let's just say for the heck of it Saruman did know who had possessed all the Elven Rings. That doesn't mean he told Sauron, and I actually doubt that he willingly did.

Sauron may have come to read Saruman's thoughts and minds; however I doubt Saruman would have willingly told Sauron who had the Elven Rings. The relationship between Sauron and Saruman was a tense one. I don't think Tolkien intended this to be an allegory; but I think the best example of explaining the 'alliance' between Sauron and Saruman would be to explain the one between Hitler and Stalin in WWII.

See the Soviets were in a tight spot, because nobody liked the communists at this time. Stalin first tried to make an alliance with France and England, but they told Stalin to buzz off. Then Hitler came with an offer to Stalin. Hitler wanted to avoid a two-front war again, so he gets Stalin to sign on to this 10 year non-aggression pact, where both sides agreed not to attack eachother for 10 years. Hitler wanted to deal with England and France first, then he would wheel around to attack the Soviets. He didn't want the Soviets to open a second-front on him so, he signed this non-aggression pact with Stalin.

Now Stalin knew Hitler wasn't a man of his war and hated Hitler probably as bad as Hitler hated him. So needless to say Stalin knew that Hitler was going to go back on his word and he wanted to get prepared for when Hitler does. The secret part of the agreement was Germany would invade the western half of Poland and he would let the Soviets take the Eastern half; this would create a little buffer-zone between the two countries. Bottomline is their relationship was tense throughout the entire war...Hitler didn't like the communist Soviets and the feeling was mutual as Stalin knew Hitler was going to turn around and attack him.

The Saruman/Sauron relationship was quite similar. Saruman was budying up to Sauron as Saruman had his own plans and he knew that if he joined with Sauron it would be the easiest way to accomplish what he wanted. Saruman was secretly planning to backstab Sauron:

"A new Power is rising. Against it the old allies and policies will not avail us at all. There is no hope left in Elves or dying Numenor. This then is one chioce before you, before us. We may join with that Power. It would be wise, Gandalf. There is hope that way. Its victory is at hand; and there will be rich reward for those that aided it. As the Power grows, its proved friends will also grow; and the Wise, such as you and I, may with patience come at last to direct its courses, to control it. We can bid our time, we can keep our thoughts in our hearts, deploring maybe evils done by the way, but approving the hight and ultimate purpose: Knowledge, Rule, Order; all the things we have so far striven in vain to accomplish, hindered by our weak or idle friends. There need not be, there would not be, any real change in our designs, only in our means."~The Council of Elrond

Saruman was secretly planning to get the Ring for himself so he could become the new 'Lord of the Rings.' And it's safe to say Sauron knew what Saruman was up to; he was just trying to take advantage of Saruman and 'milk' Saruman for all he was worth:

'Saruman is a fool, and a dirty treacherous fool. But the Great Eye is on him.~The Uruk-hai (Grishnakh)

'But they shall help to rebuild Isengard which they have wantonly destroyed, and that shall be Sauron's, and there his lieutenant shall dwell: not Saruman, but one more worthy of trust.'~The Black Gate Opens (The Mouth of Sauron)

Saruman was kind of in the same boat as Stalin, no one really liked him. England and France first denied an alliance with the Soviets, then when Hitler attacked the Soviets, it was kind of an 'the enemy of my enemy is my friend' sort of deal between England, France, and the USSR. The relationship between the three powers (in which the US would also enter in) because a tense one. And after WWII Stalin the sides would all hold a grudge. Stalin had repeatedly begged England, France, and the US to open up a second front and relieve some of the pressure Hitler was putting on the USSR. Eventually they did open up another front, but Stalin thought it was too little too late for his country and he would hold the three countries in contempt after WWII which pretty much immediately after came the Cold War. Throughout WWII nobody really liked Stalin and the Soviets.

Saruman had ticked off the 'good guys' of Middle-earth and he had ticked off his 'ally' Sauron. Though I stress that like the Hitler/Stalin pact the Saruman/Sauron relationship was tense and each one was trying to deceive the other.

Ok, so now all that info is out of the way...eventhough both Sauron and Saruman had a general agreement to work for eachother during this 'War.' They weren't on the best of terms, both were trying to trick the other one. And I doubt Saruman would willingly give information to Sauron about the whereabouts of any of the Rings of Power had he known. Sauron would of had to sort of read Sarumans thoughts and mind in order to get information that Saruman knew.