Take it from every perspective you can. From your point of view, they're fake because they act differently around you and others. They lie to you but it's a different story for someone else.
If you look at the person who is "fake". You can maybe see if they are trying to protect something. Maybe protecting themselves. Hiding something they don't want you to know. It could protect you sometimes.
But you shouldn't really care about others actions. Just be yourself and its all that matters. :) Hopefully I was helpful. Good Luck!