well that's the whole question isn't it? If you know how an open string is supposed to have, that's what people call absolute pitch?
I know the Super Mario Bros. theme starts on an E, so I can identify an unknown pitch by recalling that theme and comparing using relative pitch. But that's quite a slow and unintuitive process, and it's easy to make a mistake. People with absolute pitch just hear the pitch without having to "recall" a reference note to compare to like that.
So I have the latter, but not the former (i.e., no perfect pitch). The difference is, I can choose not to observe it. It reminds me of how I've studied Japanese for 15 years, but I can still sometimes choose not to read certain kanji if I glance at a legible word — the same is not true for English, if I see something even for a split second I've already read it.
If anything I think this reinforces what I'm alluding to.
https://www.kcl.ac.uk/research/psilocybin-research-pitch-stu...
I learned perfect pitch well after six. I know others who have too.