When a woman is cheating, she obviously wants to keep her boyfriend from finding out. The only way to do this is to be more secretive. If there are parts of her life--like going to work or the gym--that your girlfriend has suddenly stopped talking about, it can be a warning sign. It’s also suspicious if she reacts defensively or angrily when you ask her where she’s been. Overall, if she’s started being evasive about where she’s going or whom she’s spending time with, that could be because she’s going out with someone else.

Another sign that your girlfriend is cheating on you is a drastic change in her appearance, or her attitude toward you. If she starts dressing better, warns family therapist Sheri Meyers in her Huffington Post article "Is My Partner Cheating On Me? 7 Red Flags" it might be because she's dressing up for a new man in her life. Be aware if she's bought new clothes, especially new lingerie that she doesn't wear it for you. You should also pay attention to changes in your sex life; if she is less interested in intimacy with you, it can be a sign that she is intimate with someone else.

If your girlfriend is cheating on you, she might show it by projecting her bad behavior onto you. She’ll do this by turning the tables and accusing you of cheating on her. This is intended to blind you to her actions, says relationship counselor Kim Leatherdale in her article "Accused of Cheating But You're Not," on Creating Rewarding Relationships. If you haven’t given your girlfriend anything to be suspicious about, but she is constantly acting jealous and asking if you’re seeing someone else, it’s a sign she might be the one straying.

Many women use modern technology to facilitate cheating, according to Indiana University researchers Jason L. Dibble and Michelle Drouin. Their 2014 study “Computers in Human Behavior: Using Modern Technology to Keep In Touch with Back Burners,” showed that as they lose interest in their current relationships, many women keep in touch with potential boyfriends through texting and social media. Your girlfriend’s phone and computer usage might be a clue she’s cheating on you. If she seems to be texting or checking her messages more often, particularly if she is careful to keep the screen hidden from you, it can be because she’s cheating.

The right plan can help you catch a cheater, says former FBI agent Mark Safarik in the Men's Health article "8 Ways to Spot and Avoid Big Trouble." He suggests buying two identical romantic cards and sending one, unsigned, to your girlfriend's workplace. If she is seeing someone else, she won’t be sure if the card came from you or him. If she doesn’t mention it to you, show her the other card and ask her if she’s seeing someone else. The more agitated she is, the more likely it is she’s cheating on you.