There are some indications that you will notice if your girlfriend is interested with someone else. One is she is no longer excited to see you nor interested in you. She has lots of reason every time you asked her for a date and always avoiding any communications with you.
The only way to find out if your girlfriend is interested in somebody else is to ask her. Tell her you need to know the truth and that it is important to you that you find out from her and not somebody else.