A very similar expression to the one I mean can be seen here (the girl at 1:38).
Mouth slightly open (upper teeth showing), eyes almost or at least slightly closed, head also slightly tilted to one side, and a small head shake.
What's the name for this expression, or a way to quickly describe it?
"What?" she asked as she shook her head in disbelief.
(Don't think that's accurate though.)