Question: Is there a way that i can find out whats happening? I mean is there a way that i can find a track that she uses another SIM card time to time. Do SIM cards leave any marks on the phone, in the documents or somewhere?
Well, there is indeed a way to get notified when a new SIM card has been inserted into an Android device.
Some manufacturers has it built in their phones, some antivirus softwares have this feature. There are also many apps on Google Play that provide this functionality.
I don't know if it is legal to share such softwares here or not. Moderators can tell if so or not and I'll share some of them. Anyway, Google is your best friend.