Well, the same way you know if the big guys are telling the truth - you listen without trust until you can check enough verifiable claims to make up your mind.
The difference is that when a big guy lies/misrepesents/is mistaken and gets caught, it causes quite a commotion even if you personally didn't catch that particular wrong fact/report. With the little guy, much less attention is paid so if they are mistaken, or change their ways, or w/e, you can easily miss it.