Suppose we are confronted by a stamp issue that is not documented in official publications of any kind. And no, those goofy "certificates of authenticity" you see for several dodgy Russian Civil War issues don't count, for obvious reasons.
There might be some references to the stamps in contemporary publications, like newspapers, but I advise against looking at contemporary philatelic publications as being very useful. All too often, those publications were the victims of unscrupulous correspondents who had something to gain by an issue looking respectable, and an article in a philatelic magazine conveyed a lot of respect!
Next step: look for convincing postal usages. That may or may not be difficult or impossible. The only postal usages may have been on internal postal forms, or on items which had a minute survival rate. And even postal usages which look convincing really need expert opinion to be assessed realistically. As an example, here's a fake Nikolaevsk-na-Amur'e cover, offered on Delcampe in 2010:
It takes a fair bit of specialist knowledge to spot the problems with this cover: it's a fake. That even alleged experts can be fooled is demonstrated by this certificate:
It's a silly forgery, one of the many forgeries that were circulating in Moscow during the 1990s. But it fooled this Mr. Hovest, who at one point was an accredited expert for Russia!
So do we give an issue like this the benefit of the doubt and presume its innocence?
I believe we should not. I think we should regard every even slightly questionable issue as guilty until proven innocent, and proving innocence is and should be difficult! For me, the NNA issue is still guilty...