You're being needlessly dismissive.

From a philosophical perspective, there's no way to know that any piece of software is truly correct without formal verification.

But in the present, non-philosophical context, it's obvious that what we mean is, colloquially, "how well-tested is this against a variety of edge-case files which the official winrar handles correctly? Is there a test suite, and how robust is it? Plenty of software that claims to be compatible with the rar format, doesn't actually successfully read all rar files."

It's also equally obvious, in the present context, that we would prefer these steps to have been taken by the author of the software before we install it and run it on our own computers and data. The parent commenter wasn't just asking about the software's correctness for the sake of academic curiosity.

The post mentions the existence of an extensive test suite, which you can peruse for yourself if you're so inclined: https://github.com/bitplane/rars/tree/master/crates/rars-for...

I don't know how all these test cases were generated, but at least some of them seem to have been copied (with attribution) from the test suites of earlier FOSS RAR implementations.

The ideal would be to test it against a representative corpus of real-world legacy RAR files, but I'm not sure where you'd find one.