What would actually show whether this tridactyl specimen is authentic?

I’d separate the claims. “Well preserved” should come with clear documentation of the specimen’s condition and how that was assessed. Authenticity is a different question, and would need transparent provenance and independently checkable analyses, not just a striking appearance.

The sex estimate needs its own explanation too. A group saying they believe it is male tells us their conclusion, but without the criteria and supporting observations, readers can’t judge how strong that conclusion is.

For now, I’d treat both details as claims from the researchers, not settled findings. A useful next step would be a public report showing the methods, images, and any test results, with enough detail for others to evaluate them.