Some ways to know if your partner is flirting is the way she looks and acts around the person. Most often her body language will give an indication. How is she acting with you? Is she giving you the same full attention as she usually does or is she distant and disconnected? Will she sit next to the person given the opportunity? Is she starting up a conversation with someone else and wanting to continue it? When she’s talking to the person, try to interrupt and depending on how she responds will determine one way or the other. If she is happy to get up and talk with you, then you’re probably okay. If she’s hesitant it’s more of a question.
Flirting is defined as acting as though sexually attracted to someone, but playfully rather than with serious intention. But to me, if she is in a relationship with you, she should not be flirting with anyone else. If she is behaving in a way that makes you feel that she is interested in someone else, that is flirting. Speak to her about it and explain that it upsets you because you love her. Some examples of flirting include looking at someone very often, leaning towards them when they are talking, constantly touching someone, fiddling with her clothes or necklace, amongst others.