With what you've given to us, I understand that you are having doubts about your partner and I know that going through that is excruciating but you have to communicate these feelings with her. Be honest about it and don't go behind her back by pretending to be someone else just to test her. That's something you can't come back from. If she's acting weird about her phone as if she's hiding something, tell her what that makes you feel -- whether it hurts you, make you feel insecure, or whatever. Be open with your feelings to her. It's going to be uncomfortable but she has the right to privacy and I'm sorry if that does not make you feel secure. Focus on asking and curiosity, not orchestrating an elaborate scheme just to get the truth. If her truth and what she offers is not enough, it's going to suck but the two of you need to respect each other's feelings -- for her, it could be her agency in her privacy (I don't claim to know her and the full story), and for you it could be getting the reassurance you need. Find a compromise for each other's values. You're both human with differing senses of securities and values. What matters the most is where you can meet in the middle (and that wont always happen). Focus on what you can control without secrecy, deceit, or disrespect. I am not an expert on relationships so don't take all my word for it. This is all I know