How can you tell a valid opinion from a bias opinion? I realize when they are obviously slanted, but what makes an opinion valid?
An opinion is generally biased, but it could also be valid. For instance, in my opinion, a certain candidate would make the best president. I could cite you facts and statistics to back my opinion. Your opinion about a different candidate is equally valid because it's your opinion. But, there's no way of knowing now or in the future who -- of the 8 or so leading presidential candidates -- would do the best job.
I also have opinions about religion, ethics, and fashions. They are valid for me -- but they may not be valid for you. Our opinions are based on our personal experiences. Your experiences are different than mine.
When you read or listen to another's opinion, ask yourself if your experiences are the same or different from theirs.