Depends how you use the word doubt.
Do I believe with 100% assurance that Assad govt did it? Of course not.
Is it the most likely probability? Yes. Call that doubt if you want.
Should we prove it first? At this point I would say yes, build a strong case. If we weren’t going to retaliate against Syria for use of chemical weapons within the first 12-24 hours, we now need to build a coalition of support for punishment.... unfortunately.
Has anyone bothered to ask the Syrian people who they think did this?
We should prove it’s Assad’s fault before we get mired in a conflict that has the potential to be very deadly. I think Putin is all in. I think the Iranians are biting at the bit to take Syria. Now the Turks are involved. This situation is about Middle Eastern powers wanting to ascend to Great Power status.