How can you evaluate a site for possible bias?

If the site shows hateful words/comments, that is the easiest sign. Also, showing distaste towards certain group (race, gender, political party, sexuality, etc.) is a sign of bias.

Generally, bias can be quite obvious in texts and even in those with more subtle bias can be pretty easily spotted. If a site only mentions one side of a topic and doesn't analyse or even mention the other side/s it contains bias. If it shows strong negativity towards one side it is also biased. Basically anything one sided is biased so if your skimming through and notice any of these things that's how you can tell