There will never be indisputable evidence. It is in the very nature of today’s international relations that no evidence is not in dispute. One may find beyond dispute that people were gassed but one will never find a unanimous verdict as to who did the gassing. The Russians don’t mind a few hundred thousand deaths, they didn’t in Chechnya so they won’t mind at all in Syria. Their spin doctors have numerous strong rhetorical fallback positions so that they don’t have to side with the west.
The Russian public too is convinced that what’s happening in Syria is a proxy war between Russian and American interests. So even if the evidence is beyond dispute it won’t compel them to believe that therefore intervention against the regime is warranted. Assuming Russia can be swayed is a bit post hoc.
In fact, Russia has crushed the Chechen rebellion in a similar fashion in two wars, which like in Syria involved the destruction of cities like Grozny. Diplomatic pressure on Russia to relent will have to be much more intense, but unlikely to yield results. China’s position is rather similar to Russia; it’s repression in the western provinces also will compel them to protect Syria’s methods.
If the west wants the bloodletting to stop, the west will have to intervene. And for various very good reasons, we won’t. But there are measures short of intervention. Still anything forwarded in the UNSC that involved military action such as a no-fly zone will see vetoes by Russia and China.