The only way is to use forensic tools and perform a side-by-side comparison. You will need to have an original and a supposedly re-encoded version. Once encoded and altered, the 'new' video can be re-encoded to another codec, assuming new properties. I Hope it helps, seek for forensic video tools.

Sorry if my answer wasn't up to your standards, it's just that your question itself is very confusing. What do you mean by "some other x264"? Could you perhaps word it a little better? People here also have no idea what this "AI" you're mentioning has said.

EDIT: Did you perhaps mean something along the lines of: "How to tell if a remake/re-encode has been re-encoded to AV1 or some other codec?" I'm really trying to understand what you're asking. Maybe an example would help...

Thanks, but AI already mention mediainfo and about the tag from nyaa will not apply to downloaded shows.
About your statement "The codec (HEVC/H.265/x265, AVC/H.264/x264, AV1 etc) don't really matter...", is not true, it matters for some of us and that is the point of the question.

Please, if someone else has a more academic answer, feel free to comment, thanks.

The "Remake" tag on nyaa (which looks like this here) is the first indicator that a release was re-encoded more than once. However it's not always respected, so the description is just as valuable to tell if it's a re-encode. And you can check the mediainfo for any clues of leftover metadata giving out which release was used as a source, like the track names. The codec (HEVC/H.265/x265, AVC/H.264/x264, AV1 etc) don't really matter since you can freely encode from one to the other.

How to tell if a video HEVC was re-encoded to AV1 or some other x264?
Please don't paste respose from AI, i would like to know from personal info or technique.
Thanks