grammar - How to differentiate between "a clause" and "a question"?

I have been taught that when a wh-word is a subject of the question there is no need to use auxiliary before the main verb unlike the practice of subject-auxiliary inversion generally followed in making interrogatives.

So my question is how would you differentiate and tell whether it is a clause or a question. For instance, We were asked:

"Which one of us wanted coffee"?

And if I were to narrate it to someone else, I would say-

My friend asked which one of us wanted coffee.

Now the same set of words is working both as question in former and subordinate clause in the latter sentence. Since there is no alteration because of no subject auxiliary inversion how would I know whether it is a question or a clause?