I hate to say this because it could come off sounding sexist, but let's face it, women are great at catching guys when they lie. It is just a fact. So don't bother lying to a women. She might not catch you today or tomorrow, but one day she will catch you in your lie. Even worse, she might not bring it up immediately. She might just stew in her anger and resentment for a while, and then add that to a list of complaints she has against you one day when an argument breaks out. Just don't lie!
If you want to go out with your friends one night, just admit you'd like to go out with your friends. Don't lie and pretend you are not going out. She is going to catch you. One of your friends is going to post a comment or a photo on Facebook with you in it, and she is going to see it. Just simply tell the truth. If you are dating a reasonable woman, she will understand you need to have some time with your friends here and there. If you are dating an unreasonable woman, you might want to reconsider why you are dating her at all.
Of course, if you are cheating, that is a totally different story. Oh, one day you are going to get caught. When it happens, and it will happen, it is going to be bad... very bad. A simple solution - simply don't cheat on your girlfriend! You deserve whatever punishment she sees fit if you do.