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They ARE out to get you.
One way can be to scroll down in General - About Device - Status - Device Status, should show official. (this may only be for some versions)
Download Samsung Kies on a pc then connect via usb.
Run third party apps like Genuine Galaxy, Phone Spec, or Antutu (info), though Antutu has been shown as inaccurate.
There are other options I haven't thought of.
Good luck. I'm sure it's fine.
If you have a reason to suspect it's fake, come back.
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