The same CIA that told Bush I, Clinton, Bush II about the WMD's in Iraq? Sorry, if the president can't come up with better than "We conclude they used..." it's not selling. Not to the UN, not to the American people, looks like not even to Congress.
Well, it's not like there's any question that Syria has chemical weapons. And there isn't much legitimate question that someone used chemical weapons in Syria. So the only issue to be determined is whether the regime used them or the rebels used them. (setting aside for the moment whether Assad ordered their use or someone under his command took it upon himself to order their use).