How can I tell if a girl I've been in a relationship with is cheating or not?

Im struggling with myself here on what to do in my situation. My girlfriend is coming over tomorrow to my apartment. I was on a video call with her today and i here a male voice in the background by her bedroom door she said it was her dad getting a broom but im not 100% on that and she said bye to them as they walked away. We haven't done the deed its been maybe 3 weeks since weve done the deed. I have that gut feeling she is seeing someone else and also seeing me at the same time. I already confronted her and she said no she's not. I think she's lying but i dont know what to do anymore. Any help is appreciated.

How can I tell if a girl I've been in a relationship with is cheating or not?
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