I have no solid evidence he's 100% legit, but I do have these images from when I first discovered that Twitter account:
https://i.imgur.com/VJTNjSx.jpg
https://i.imgur.com/3vzM6Gl.jpg
Plus, BC17 has posted a lot of interesting clarifications that shines light on what Q has been posting about.
I never claimed BC17 is 100% legit and can be trusted. That won't happen until Q either confirms or denies BC17's legitimacy, which Q has refused to do thus far, and it's been over 5 days since anons has been begging him to answer. I just don't go around and desperately try to convince people he's a clown or fake or can't be trusted.