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<Google play>Please help me.How to judge if the user has bought my app?

I need your help.I’m very, very sorry to disturb you.

This is my game:
https://play.google.com/store/apps/details?id=com.FourFire.InfiniteSoul

My app is uploaded to google play. Paid downloads.
How to judge if the user has bought my app?.

Please help me. This problem has been bothering me for a long time.
English is not my native language, sorry.
I hope you can help me.