Have you washed your hair? Did you wash you hair?

[ROFTOK]

I am waiting for a friend of mine outside their house. The friend is supposed to come out soon. Finally, they come out and I see that their hair (on the head) is wet. Which question is better?

- Have you washed your hair?
- Did you wash you hair?

 

[5jj]

We'd normally use the present perfect in BrE.

There's no need to add 'on the head'. Unless we specifically mention another part of the body, we assume that 'hair' is on one's head.

 

[Tarheel]

@ROFTOK Your hair is wet.
@Tarheel I know. I just got out of the shower. I didn't have time to dry it because somebody was waiting for me.
@ROFTOK Me?
@Tarheel Yes, of course!

I don't know at what point my friend would find it appropriate to ask if I had shampooed while I was in the shower. (Americans don't wash their hair. They shampoo.) Probably never!