Obviously, as a man, you wouldn't wish to love a woman who sees someone else. I don't think it's new to you that women too can have backup lovers like most men do.
So how would you know you aren't the only man she is seeing?
1. She doesn't give you the confidence that she is yours alone
Usually, when a woman truly loves, she makes her man feel she only belongs to him and no one else. However, in the case of one who sees another man, she wouldn't have the urge to give her man the confidence that it's only him that she loves.
2. She doesn't give her all in the relationship
When a woman loves a man, she gives her all in the relationship to make it great. This is different from when she sees someone else because, at the end of it, she has someone to hold on to.
3. She demands a lot of space
She is not that interested in being with you. If she is capable of going for weeks or months without spending time together with you then she is likely spending time with someone else.
4. You engage in one-sided conversations
When she only wants you to listen to her own side of the story but never listens to yours. She wants you to go with her own flow and never agrees with whatever you have to say. This is a huge problem for you.
You no longer hold any harmonious communication with her. She will always have a reason to lash out and argue with you. She could be playing a side game if she suddenly becomes nagging for no reason.