PC Master Race

CPU-z can assist in detection. Another option is disassemble of the card from the fans, heat sink, etc. And verify the physical components vs. A known good source card.

You could run some GPU based benchmarks and compare results to other user submitted benchmarks to see if there's a massive deviation from the norm. Ymmv, unless the results you're comparing to have a similar entire system setup.

There's also nvidia profile inspector, can compare that with cpuz to point out any spoofed possible changes if they don't align data wise.

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.

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.