Are you asking me how I know I'm immune to brainwashing? Or just in general how would one know?

Personally I don't think the government is trying to brainwash me, so I have no need of immunity. I also don't think it's that easy to brainwash people.

I do believe corporations target us with a lot of advertising and even try to change culture so that we consume more of their products. That's way more of a threat to us than any government brainwashing program. I suspect we're all more vulnerable to that advertising than we realize. Most of us think we're too smart, but those corporations spend a lot of money on research on ways to get inside our heads. The solution is to turn the tv off. If I went around saying that to people, their eyes would glaze over. People largely don't care.

Have you watched The Century of the Self? It's a four part documentary. By Adam Curtis, I think. Really interesting.