Lemme rewind the thread a bit. I think the whole "who do you trust in Syria" conversation wound up kind of missing the mark and being about the general credibility levels of the Russian and American governments.
That's probably not the best frame. A lot of the information coming out of Syria is from third parties like the Red Cross, HRC, Amnesty International, etc. They're not infallible, but there's much much less of an incentive for them to mislead than there is for Russia, who's an active participant in the war and who vouched for the removal of chemical weapons by the Syrian government. Russia's been pretty predictably running a PR campaign against the "white helmets" (rescue workers in Syria who also document the war) as essentially being crisis actors and terrorists.