I don't care that my friend is asexual
Why would you need to know then?
I wonder what it is like not to have a sex drive
I can see why that would be a a curious thing for you.   You really should not pry on someones sexual preference.  If they are close to you, they will disclose that to you on your own time.
Why wouldn't I ask? They are my friend.
Oh, if they are a good friend then I feel like it's great to know them more a little better. I'm sure they'd appreciate you being interested in what kind of person they are, perhaps you'll learn a lot from them!