Talk to her?

Signs

Method 1 Looking for Changes in Your Relationship. Listen for complaints about you or your relationship. ...
Method 2 Watching Her Behavior. Notice if she’s being secretive with her phone or computer. Watch how she handles her phone and computer around you.
Method 3 Talking to Your Girlfriend. Tell your girlfriend about your suspicions. ...

Doesn’t tell you where she’s going, hides her phone from you when you’re around, avoids plans with you, gets hella defensive when you ask her certain shit, etc..

Ask her

🥴 ummmm I guess if you know her then you would figure out for real

Probably from all the audio dialogues between her and random whoever. You know the ones where you hear them fucking for hours

You don't

She pays more attention to her phone than you. ...
She's less interested in sex. ...
Her social calendar is suddenly bursting at the seams. ...
She accumulating new clothes at higher rates than usual. ...
She's overly concerned with your whereabouts. ...
She isn't making plans for the future.

Reach out to someone else that knows her but won't tell her or check her phone