Know a Lie
Before you confront the person who you suspect is hiding things from you, take a deeper look to see if she is really lying. Not every seemingly deceptive behavior is actually indicative of a lie. For example, some people believe that liars avoid all eye contact and a constant gaze means that the person has nothing to hide. This isn't always the case, according to former FBI behavioral analyst John R. Schafer on the website Psychology Today. Instead of staying on the surface, look deeper to decide what the person in question is hiding from you. Watch her overall behavior, looking for statements that don't make sense or match up to what she's previously said, or for serious omissions.
Address the Issue
You've reviewed the clues, listened to what he's said -- or not said -- and now you're ready to have the "big talk." You aren't a mind reader and shouldn't assume that you know why someone is hiding something from you without asking. Sit the other person down and ask what's going on. Let him know why you think that he's hiding something and provide any proof that you have. For example, print out the seemingly romantic email that your guy's co-worker sent to your joint address. Be assertive and clearly speak your mind. Don't allow the deceptive individual to turn the conversation around on you or shut you down. Use a brief and direct approach before he has time to think about how he will continue on with his lie.
Just confronting the person who is hiding things from you isn't always enough. While you want to tell her that you know she's lying, you also need to let her know how this affects you. For example, if your girlfriend is hiding a secret romance, you need to tell her exactly how her actions are making you feel. Focus on your feelings and use "I" statements, such as, "I felt hurt when I saw the email from that other guy."
There are some lies that you just can't abide. If whatever the other person is hiding from you truly breaks your trust, you may need to walk away. Without trust you can't have a relationship. This applies to more than just romantic relationships, but also to friendships. Whether your feelings are hurt so badly that you can't recover or your efforts to mend fences have failed, when the trust is gone, you need to end the relationship and move on.