While lots could be said about bias of all types in source evaluation, keeping these two principles in mind is a good start.
We all have values, beliefs, and presuppositions about the world. When we interact with information, we will naturally be attracted to sources that confirm our existing beliefs and distrustful of sources that challenge those beliefs. This means that as we evaluate sources we are at risk of:
When we talk about bias, our first thought is usually of a source or figure who we disagree with and perceive as biased. But there is little risk that we will unthinkingly accept the claims of a source we disagree with. There is huge risk that we will unthinkingly accept claims that "feel" right to us. No one is immune from this phenomenon, and, while it is emotionally challenging work, confronting our own cognitive bias is the best option we have for being thoughtful information users.
Even saying bias can be a loaded term, because everyone has a particular perspective. Knowing the perspective of your source is valuable context for how you use it. But that doesn't mean that having a perspective (even a strong one) means a source is inherently unreliable. Let's do a thought experiment:
Let's say I'm writing about a famous case decided by the Supreme Court and I find an article arguing that the court decided the case correctly.
So what does this mean? Learning the perspective of your sources isn't about tossing out any that have a whiff of bias. It is about knowing whether I'm getting the full picture and not becoming overly reliant on a source as though their perspective is the definitive one.