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How to identify "legit" GPU
(reddthat.com)
I would make sure GPU-Z claims it to be a 3070ti and then run the Time Spy benchmark from 3DMark to make sure it performs as expected.
A benchmark is going to be the most reliable way I think, it seems like the fake GPUs tend to reflash a really crappy one, so benchmark results from a fake one would be extremely different from what they should be.
GPU-Z can detect some reflashed GPUs, not sure to which extend. This in combination with your benchmark idea should do the trick. Or you take it apart and look up the markings on the die.
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