The only way to do that is to catch them fabricating news items (see keywords jasonblairsyndrome, fauxtography, fakephotos, etc.) and even then it didn't stop Dan Rather from proclaiming that the evidence he trumped up was "fake BUT ACCURATE!".
And at that point the journalist (and network) have been discredited. But what would be there to restore trust in the media as honest or objective?
No, you don’t have to catch them fabricating stories. All you have to do is show that they’re liars.
Do I have to explain what NewsBusters tries to do? The problem they have is that, until recently, they haven’t caught on how to do things in an effective way that reaches a wider audience. They still don’t have it 100%, but it’s clear they realize how they should be doing things. (That refers to the way their site is structured and the like, not so much the way they write their posts.)