One of the first signs that your girlfriend might be interested in someone else is a decrease in her interest in you. She might show this in a variety of ways. For example, if you used to spend every weekend together and she suddenly starts making other plans without you, it can mean you are no longer a priority in her life. Similarly, if she stops returning your phone calls or it seems as though there are huge chunks of time that she's completely unavailable to you, it can show that she is closing you out of her life and has another person on her mind.

Women tend to give physical clues when they are interested in a person. Watching how your girlfriend interacts with the men around her can tell you if she is attracted to any of them. If she's talking to a man and she is flicking her hair or tossing it over her shoulder, it can be a subconscious sign that she is attracted to him. It's also helpful to pay attention to her hands. When a woman is interested in a man, she tends to touch her face and her neck while talking to him. Another sign to watch for is where she points her knees. A woman may point her knees toward the person she is interested, effectively creating a triangle of personal space.

If your girlfriend has suddenly changed or taken a new interest in her appearance, it can mean that she is also taking interest in someone else. She might show this by suddenly changing her hairstyle and then brushing off your compliments. She might also start going to the gym more or taking additional care with her makeup. Dressing up for an evening out doesn't automatically mean she has her eye on someone else, but if she's making dramatic changes for no apparent reason, it could be because there is someone new she's trying to impress.

Indiana University researchers Jason L. Dibble and Michelle Drouin published a study in 2014 that showed many women use modern technology to keep a potential boyfriend in the background. As their current relationship starts to dwindle, they increase communication with the other person through social media and texting. Some of the signs that your girlfriend might be cultivating a backup relationship include spending more time on her phone than usual or a deepening interest in social media.