From talking to them. Brokaw and his ilk are "CORPORATIST SHILLS". They are "IN THE BACK POCKET OF BIG BUSINESS". They are "APOLOGISTS FOR THE MILITARY-INDUSTRIAL COMPLEX". They help "KEEP THE PEOPLE IGNORANT OF WHAT IS REALLY GOING ON IN PLACES LIKE ISRAEL". Some leftists even believe that the mainstream media is "CONTROLLED BY THE ZIONIST LOBBY".
I'm not making these quotes up, you know. I've actually heard them.
Just because I, and perhaps you, think Brokaw leans to the left doesn't mean that's what everyone thinks. Just ask an extreme leftist sometime.