Yeah, without the paper you can never know for sure. Just like if a person was an orphan or adopted, even though they could *look* like a lot of things they wouldn't really know what their heritage was.
It's also kind of like how many people own dogs that are generally a "mutt" but have a predominant element that makes some sort of history more obvious, like the head shape and temperament of a retriever but a different body type, or different fur type/color, etc. Many owners will say that their dog "has some [breed] in it."
Ultimately, without a pedigree that's what you get to do as well. Your cat is "mostly Persian." What's interesting, though, is that the extremely foreshortened faces don't happen in the wild -- they're only around because of breeders. So if your cat has such a face, but isn't pedigreed, it's likely that the person you got it from (or the person they got it from) got an actual persian, bred it to a cat without permission, and sold them as "persians" without any sort of certification. And, of course, people buy them and are happy with their cat and then wonder why they were charged so much money despite not getting any paperwork...
Still, a lovely cat is a lovely cat
One thing that I wonder is how old is the cat? Kittens have a generally thinner coat than adult cats; I was surprised how thin our new kitten's coat was, when we saw him at 8 weeks. Especially compared to his mom.