I'll take you through a Thought Experiment to show why this isn't really possible.
Imagine you were trying to judge whether networks A, B, and C were 'biased' on the coverage of, say, "Should the US offer aid to the rebels of [country name]?"
So, you might think, well, they should have facts from all sides. Present all the facts. Have people on to discuss any point of view on the issue. Give equal time to each side. Don't exclude any news that sheds either side in a positive light. Etc.
... now let's change those 'rebels' to a genocidal group that utilizes a child-army and practices slavery.
Suddenly, things change. If your network gives facts/opinions/equal-time to the genocidal/slaver group, it suddenly feels a lot less 'unbiased' and starts to feel a lot more like they're giving legitimacy to something horrible.
Do you now see the discrepancy? You're now in a position of having to figure out, for this particular issue: how legitimate is each side? Because you can't cover both sides 'equally' and have it be balanced if only one side has a legitimate cause. It's not like someone would expect News Co. XYZ to be balanced by giving equal time to a NASA Spokesman and a rep from the Flat Earth Society.
Let's take an actual issue: Brexit.
News Organization A covers Brexit by giving the facts from both sides, giving equal time to both proponents and dissenters. Organization B has more pro-Leave airtime/arguments; Organization C has more pro-Remain. So which ones are biased?
Well, how legitimate is each side? You could say Organization A is non-biased... except it means you've given each side roughly equal legitimacy. Is that fair? How do you tell? In any sort of non-subjective manner?
Now multiply this by the number of issues that people argue about. And... add in a dash of an uncomfortable truth: generally most popular political topics do indeed have at least some legitimacy on both sides (which a lot of people don't like admitting, since it makes it harder to viscerally hate the people that disagree with them.)
So... how would you design that experiment now? You can't - because "balance" depends on your subjective measure of each side of each issue.