Spying on your significant other is NEVER ok. It is not your RIGHT to know everything about someone or what they do. If you think your girlfriend is lying to you about something, ask yourself why she would feel the need to lie to you in the first place? How can you make her feel more comfortable being honest with you? How can you make her feel like you won't react with judgement and anger? And don't make assumptions about what she is hiding, it could be completely benign. For example, someone I knew started spying on her husband because she was worried he was having sex with someone else, and discovered that he had lied to her about his whereabouts and about a charge on his credit card, freaked out at him and accused him of having an affair, and it turned out he was just getting her a custom-made piece of jewelry and wanted it to be a surprise. Needless to say, her spying and accusations damaged the relationship and for what? So she could justify her own insecurities? People need to realize, you do not OWN your partner. Even if they are hiding something from you, they never do this to hurt you (it's generally with the intention of avoiding hurting you - think about that), and half the responsibility is on you for creating an environment where they don't feel safe being honest with you.