How exactly can you tell if fast or slow Met

Sooo, hey, my question is very broad but oh well. I was reading and watching videos of slow and fast metabolism, was well asking people at gym of course trainers so forth, about how exactly can a person have a fast or slow metabolism.

The GYM dudes said it so bro man "if you hungry all the time brah"

And a doctor "BMI determines it"

Like honestly how can you truly tell and no it is not about me reason why I wrote "broad" hope someone can anwsee my question. Peace.