Experience. Looking at reality. You should try it sometime.
For most of my life most of the media here has been unrelentingly right-wing. Almost all of the printed press, and much of the TV (including the BBC, incidentally - their news and current-afffairs department is stuffed full of committed Tories, and it frequently shows - the most one could say is that even they generally aren't as crazy extreme right-wing as the US Republican party).
Occasionally there have been demands for the Murdoch Press in particular, to be obliged to give 'the right of reply' to those they attacked. This would always be greeted with howls of outrage by the right. Because "free speech" doesn't actually mean you let the other side speak, it means those who own the means of speech, predominantly rich people on the right, get to decide what gets said.
On top of that you had the BBC for many years having a special MI5 officer whose job was to vet anyone they employed to ensure they weren't unacceptably left-wing. You had organizations like the "Consulting Association" that would keep registers of building workers who were known to have the wrong, overly-leftist, political views, so employers could refuse to employ them.