Have media companies declare political affiliation when registering as media.
Everybody has biases. Force them to admit this when writing "news". That way, you can call ignorant bigots just that, as it highlights the fact they ONLY read "news" from one side of the political spectrum.
Diversify knowledge and watch the leftist cult crumble as they can't survive a population of independent thinkers.
Force them legally. I'm a little ignorant in the intricacies of laws regarding media operations but if the republicans grew a spine, couldn't they in theory force their hand? It avoids the dangerous precedent of trying to FORCE them to tell the truth, rather instead highlights their inherent bias of not telling the truth...and hopefully the American public is smart enough to recognize that so long as it's pointed out to their faces that one side tends to lie more than the other.