There is ZERO evidence that a trial would not be fair. Like it or not, our criminal legal system works just fine and generally produces the right results. If anything, our system favors the accused and we let a lot more people walk who did it than punish those who didn't. Snowden would be fairly tried.
There cannot be "evidence" of something that hasn't happened yet.
There are hints though, and opinions from knowledgeable people, that he wouldn't have a fair trial, for he'd be tried using a law intended to deal with spies, not whistlebowers.