There is strong circumstantial evidence, but proof is a slightly different game. It would be nice, for example, to have a clear motive for why Russia might have poisoned the man. According to the wiki article the Russian justice system had sentenced him to 13 years in a penal colony in 2006, and then handed him over in a prisoners exchange in 2010. What happened to upgrade that to a death sentence in 2018 that the GRU didn't know about in 2006-2010?
Evidence should be a high bar. We might suspect the Russians, but lets not pretend there is any clear public evidence beyond government say-so. The government can be wrong, just like the rest of us.