Reading a character I know is telling the truth

I’m playing this Apocalypse World play by forum on an Italian forum and that’s what happened to me. Marshall, the operator, has just been shut out in his technical work. Ellen, the savvyhead, was helping him to fix this old tractor.

Marshall go and see Ellen to inform her that their gig has just gone to hell. On that occasion, while he’s talking to her, he also reads her, but he gets a miss and I, as the MC, decide to do one of my moves. I turn his move back on him, so I tell Ellen: “Ok, he comes to you with that ass face, but the truth is he’s an open book to you: it counts like you’re reading him with 10+, so you hold 3 and you can use them as normal.”

Now Ellen’s player asks me a question nobody’s asked me before, while playing Apocalypse World. He asks me if he has to use one of his holds to ask Marshall “is your character telling the truth?” in order to know, as a character, whether Marshall is lying or not. The fact is obviously Ellen’s player, as a player, knows Marshall is telling the truth.

And that’s my question: how should I handle this situation? What should I answer to Ellen’s player?

Ellen is unsure about you and wants to discern your motive? Give her that and display the contents of your heart; disappointment that Ellen is suspicious of you, frustration of failure, fear of the consequences, lets crack open the lid of Marshall's poker faced desperation.

I don't understand what the confusion is about.
There's a line between what you know and what your character knows. 

If the player already knows, then no, don't bother spending hold to ask. Ask something you want to know instead.

This is true almost all of the time. There are times when the player might choose to spend hold to ask anyway, just to be sure that the character's knowledge is formally acknowledged and ironclad, but most of the time you won't need to.

-Vincent

OK, thanks to all of you!

While I was still waiting for advice, I temporarily answer to the question saying to my friend: “In my opinion, you shouldn’t spend the hold, and the reason is that’s not really a question you are interested in (you know the answer yet).”

So, I’m happy I quite got the point, since part of Vincent’s answer seems close enough to what I managed to understand on myself.

Exactly so!

-Vincent