How exactly was this determined to be true, and by whom?
It sounds like he admitted it to the committee. Read the articles that I have posted, including the one from National Review. They're all in this thread. Also, google the names "Marc Rich" and Libby. There's a lot of stuff, from reputable sources, that show the close ties between these two jerks.
Congress had the phone records. I watched Libby's testimony on C-SPAN. He acknowledged making the call. At the very least, he exercised very poor judgment given his position as COS for the VP. Libby stopped being Rich's lawyer in the Spring of 2000. Obviously, they had a personal relationship that transcended their business dealings. You should read Libby's testimony before Congress.