A nerve agent made in Russia - that's what we are told. I would believe this only if confirmed by a truly independent third party.
I vividly remember the US presenting "evidence" in the UN to start the war in Iraq. None of the allegations held water.
How about holding these affairs to the same standard as crminal trials? Means, having a verifiable chain of custody, presenting the evidence to public scrutiny and having each side challenging the evidence.