What should you look for to determine whether or not a site is biased?

Personal opinions and one-sided answers.

Open-ended or negative comments or conversations would be first on my list.

A good example, I would say, is a celebrity's Instagram account. Most people who share comments on the person's page find positive interest toward the person and feed off each other positively, while others go there for the opposite reason. Generally to make the person feel bad about themselves and people who feel the same way agree with them.