Edited for understanding. please follow the rules for chat/text.

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How I will I come to know that my ex girl friend still loves and thinks about me even though she is seeing some other person. And how long should I wait for here to return please help I can't be without her

If she is seeing someone else than she is sending you a clear message that she wants to be with this other person.
You best bet is to move on with your life and understand that not all relationships last forever.
For your own peace of mind it is never good to hang on to something that is no longer viable.

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Read the rules for chat/text/spelling, so you won't get deleted and we can understand your question.

If she is seeing someone else, then I hope you leave her alone, and get your own life together, and do your own thing without her.

Don't wait on her to come back, because she might not want to, sorry.

Its is pretty clear, lets ellaborate a little, IF she love you, she would do what love takes, she would love you with her actions and words. What would you do if you love someone? That's roughly the answer brother.

Its possible for a girl to still love her ex even though she's moved on with someone else..
Just because you have feeling for someone else, doesn't mean you have any more or less feelings for another.
If you believe it and focus on it, she'll be back.
It always works for me..
But you must be patient. It takes time..
Whatever you do, don't make it show that it phases you what so ever that she's with someone else. Casually talk to her, if anything act like your happy for her.. and move on yourself.
Seeing YOU with someone else will realllyyy get to her if there's any potential.
And who knows, maybe you'll find someone better in the process..