We have not seen any actual evidence and, as Tariq Ali pointed out, on Democracy Now:

“Virtually no one who knows the region believes that these attacks were carried out by the Syrian government, or on its orders,” he says. “It’s crazy, if you think about it. They let the inspectors in, and then in a hotel barely 10 miles from—in a location barely 10 miles from where the inspectors are staying, there’s a chemical attack. And what good does it do the Syrian government to actually open fire on these inspectors? They want them there. So, I think it’s slightly incredible. And given that citizens in the United States and Europe were lied to in the run-up to the Iraq War—simple, straightforward lies—it’s very difficult to take the West seriously when it cries wolf again. So, till the evidence is there, it’s impossible to take this at face value.”

Can we believe our own government? Had the Iraq war made it impossible to ever do so without having doubts ever again?