You can tell someone something that is a fact and not an opinion. If they do not want to believe it they may say "you are a conspiracy theorist" then you can provide them real verified video evidence of your claim. You could even provide them their own source to prove to them the video is real. And they will turn their head and say "Im not watching it this is crazy"
I will say this is a big problem with democracy I say this even though I do believe in democracy it's the best system for us in my opinion. The simple fact is many people will believe what they want to believe. I don't like to question peoples intellect for it I believe its much worse than that they are stupid. I think being closed minded is much worse than being stupid.