I cannot - nor can anyone, really - demonstrate they are a native speaker. I can merely say that I am. I also happen to be a linguist and could provide criteria for that (please see my link below.)

Generally though, one doesn't claim to be a native speaker when one isn't.

So, the question is this: How can one tell if those YouTubers are in fact native speakers?

I answered this question in another forum and it was accepted by the OP.

I'll give you the link but do not wish to post the entire thing here:

native speaker and idiomaticity

I cannot - nor can anyone, really - demonstrate they are a native speaker. I can merely say that I am. I also happen to be a linguist and could provide criteria for that (please see my link below.)

Generally though, one doesn't claim to be a native speaker when one isn't.

So, the question is this: How can one tell if those YouTubers are in fact native speakers?

I answered this question in another forum and it was accepted by the OP.

I'll give you the link but do not wish to post the entire thing here:

native speaker and idiomaticity

Basically, what can be said of native speakers applies to those who are fluent in a language even though a native speaker may have "more fluency" that a fluent non-native speaker. The basic premise is the same.