Of course people lie, and they could easily lie about how they're feeling. But what possible basis do you have to argue against what someone else says they're feeling?

If I tell you that I'm feeling hungry, for example, how could you possibly make an argument that I'm not?

You could see that I just ate a sandwich, but that doesn't mean I don't still feel hungry. In fact, you could see that I just ate 10 sandwiches, but it's entirely possible for someone to still feel hungry, based on how the brain and human psyche work.

The best case arguement is that a person's actions are seemingly inconsistent with a certain stated feeling: for example a widdow who says that she's crippled with sorrow, only to be caught going on dates with other men. But again, you're not arguing feelings there, you're arguing an opinion about the consistency of behavior.

The feelings of others are fundamentally unknowable to us. Expression (words, facial expressions, body language, behavior, etc) is our only window into the feelings of others.