You are looking for the anger micro-expression (I strongly disagree), sadness micro expression (we will be separated), disgust micro expression (he fell so low) or, perhaps, contempt micro expression (I am better than that), I guess.
Other signs will definitely vary depending on the person, you need a baseline. It could range from disengagement like leaning away to a higher pitch, a change in the pace... But none of this alone is a sign, you need several.
I won't try to do the lie detection course in a few lines
I think you may see surprise first, then if you see anger, disgust or contempt that's a strong warning. If you see happiness that's a good sign, she is happy for you. If you see sadness, it could be either "this won't work" or "it will be tough (but we can do this)".
That said, given the stakes, I would like someone to vouch for my answer or add to it.