By checking for opinions, and grammar errors. If the website has those 2 things it is not credible. Also, most .com websites aren't credible if those 2 things are on them.
If the website is free of errors or opinions, and it does not look biased, the site you have is credible.
I just did a research paper. I used this information that I learned and I ended up getting an 100% on my paper.