Relating to your partner when she is upset isn't easy; it's even harder when she doesn't tell you that she is upset. Fortunately, most people cannot fully hide their emotions and display subtle signs that reveal their true feelings.
One indicator to watch for to help you determine whether she might be upset is her body language. People's body language changes depending on their mood and inner emotional life. If there is something bothering her, her posture may change. She may cross her arms defensively and keep her eyes downcast. She may not look you in the eyes as often. If she is upset with you, she might not reach out to hold your hands as often as she used to do.
Another means of determining if she is upset is to listen to her. What she says and how she says it changes depending on her mood. If she is generally speaking less and also laughing less she may be upset. If she is upset with you, her tone of voice also may change when she speaks to you. She also may be short with you and reply or talk to you in short sentences without elaborating.
Whatever it is that is upsetting her, she will try to avoid discussing it. She may be avoiding certain topics of conversation or change the subject prematurely. If she's upset about her weight or a comment someone made about it, she may avoid certain foods. Depending on what is upsetting her, she may even avoid you if you're the reason she's upset.
An upset woman may be more affectionate than usual and seek comfort in her partner. While everyone is unique and doesn't behave the same way, if she is upset about something, she may seek to be comforted by you. She may hold onto you longer or tighter when giving you a hug. She may snuggle up to you in bed. However, if she is upset with you, then expect the opposite -- less affection.