Question

in your opinion, is it possible to guess other people's feelings by observing their facial expressions?

Answer 1

Yes. We can know if a person is happy or sad through a smile or a tear.

Answer 2

I think it's easier to guess their mood (angry, sad, happy, tired) by their facial expressions but I don't think we can guess someone's feeling on the inside exactly just observing their facial expressions. 

(tradução: Acho mais fácil adivinhar o humor delas (brava, triste, feliz, cansada) pela expressão facial delas, mas não acho que conseguimos adivinhar o que uma pessoa sente por dentro exatamente apenas observando sua expressão facial.)