How to know if my girlfriend is cheating on me?

hey guys! im in a long distance relationship with my girl whos colombian and i met her in france. shell stay there till december and we said we could meet up three times by that time. i met her in france in july and was with her 4 days. we been talkin since then everyday we both love each other she would tell me that everytime we talk but its been 2 weeks now or more i text her and she would answer me 8 hours later ( im livin in greece its only one hour aheaD). she says im the love of her life but she doesnt show that watsoever..we r both 17 and she says that if i dont doubt her about stuff she would stay with me. because for example a friend left her a facebook comment yesterday that he ''loves'' her and that she could b the ''love of his life''. she said they b doing that for 4 years and they r joking with each other...anyway girls ladies.. please tell me whats really going on with her? sometimes i think that it could b her social and loose character..