Could I ask my asian friend if he knew any martial arts?
Why do you want to know if he knows martial arts?
I heard about this one technique that blocks internal attacks and I want to know if he knows how to do it.
It's great that you want to learn about martial arts, but it's inappropriate to assume that your friend knows martial arts just because he's Asian. That's a stereotype that just isn't true. I don't think you should ask him, because he might find it offensive.
I thought it's just one type of martial art.
It might be, but I still don't think you should ask him about it, unless you know that he practices martial arts. It might be best to talk to someone who you know for sure is interested in martial arts.