I think that's simply not possible without having insider information themselves. For example I have family members, who got information of car manufacturer. One is a slightly higher up and another one works for a company developing software for a car manufacturer and sometimes gets brought over in their R&D department to work locally. He has all the access to the databases and even walked through the buildings where future cars are planned and so forth.
So I probably could get vetted, even though I don't know what the vetting process entails. But as I understand it the vetting process doesn't proof the creditably of the information. It's just there to proof "Hey, the person could have information, because he has access to X or knows Y. ". That seems to be the extend of it.
I think the solution has to be a tier system or something else, which tracks their track record without using any loophole like plans can change. Make no mistake they can, but it's difficult to know what info changed and what didn't, so a tier system would be redundant. So with that said, I would like something along those lines...
- Vage information (something is happening in the next 50 years at Nintendo) -> 0 points
- Semi detailed information -> 1 point
- Detailed info -> 3 points
- Got information wrong -> -2 points
They than get put into tiers depending on their point and if they got less than 0 points after being tracked for 5 rumors, then they'll get banned or at least un-verified. I think banning is harsh.