(*ok, not everything....)
Ok, so, as of the end of "The Package", we know that Widmore brought Desmond back to the Island, presumably to stop Flock (probably "from leaving" and possibly "once and for all")
So, how does Widmore know Desmond's "special"?
I thought, "perhaps he always kind of knew..in the same way Eloise knew. He knew Desmond was special and would 'one day be needed', and THAT'S why he wanted Desmond away from Penny, because he didn't want his little girl heartbroken or her life torn apart when her boyfriend/husband did what the Island needed of him"
To me, it worked, but it was a bit of a stretch and a lot of it was taken on faith, but today I had an epiphany as to exactly how Widmore knew...
"If anything goes wrong, Desmond Hume will be my Constant"
It was all in Faraday's book.
After Eloise shot and killed Daniel, she used his book and followed it to make "Whatever happened, happened". Yes, her and Widmore were once an item and appear to have had a falling out, but, as we don't know when this happened, it may be safe to say that Widmore was aware of some (if not most) of what was in Daniel's journal.