How do you noe if ur frend is a good frend or a bad one?

My frend cant lie to save her life ysterday she told me she didnt have a fbook she was too young then to day she added me as a frend but she told me yesterday she never had one but she had post since december 2009 and now i hate her bff shes a backstabbing slut and she always lies that her bff from palm beach is visting her but i went on fbook and it said hanhing oit with ---- and she told me it was a girl named sabrina but she put hanging out with brianna an idk should i tell her to stfu and leave me alone or she just is really stupid an is a bad liar but shes my frend