Thank you for your reasonable reply. As I see it, there are a few ways bias can manifest. The most obvious is in reporting things that aren't true. One step down from that is reporting things that are technically true, but in a misleading way. (You definitely saw a lot of anti-Trump bias manifest in that way.) The next level from there is choosing which truths to report, in order to paint a picture. That's the level that I personally have seen most often on Breitbart/Drudge. You may well be right that they dig up uncomfortable truths that are ignored by other sources. But how often does one of those uncomfortable truths lean toward painting liberals in a better light than conservatives? It's like when negative results of scientific studies are thrown out; we can end up seeing a pattern in the resulting aggregate data that doesn't really exist.
Then you also have to look at the tone in which things are presented. For example, at the top of Breitbart right now: "At Last: The Lena Dunham Meltdown You’ve Been Waiting For ‘I ached in the places that make me a woman’" There may not be anything factually inaccurate in that article, although I haven't checked all the quotations myself. But it seems pretty clear that this is a pretty shameless gloat. I mean, they acknowledge it as such in the headline. I agree with you that there's nothing wrong with looking at varied sources, if you do so critically. I would suggest looking at an article in Breitbart, then finding reporting on the same thing from a more mainstream (call it left-leaning if you want) outlet - say the Washington Post or the NY Times. Compare the statements of fact, sure, but mostly compare the tone of the articles and ask yourself what picture they're trying to paint. Obviously what comes off as neutral and what biased would depend on where you yourself sit on the spectrum, but regardless I think it's a valuable exercise.
To take it further, where the two sources differ significantly, check facts. And it doesn't mean just find a source to corroborate them, as there will always be an echo chamber of sources repeating the same things on both sides. Instead you need to dig a bit deeper.
The easiest example is a quote. It's amazing how often people are misquoted, or the quotation is interpreted differently than I would view it. Very often these days you can find video of the original quote, and interpret it for yourself.
Now, the veracity of a statement is a bit more difficult than just whether or not someone said something. Often you can start by checking wikipedia, then look to the sources it references, for a good start.