There are many things to consider when deciding whether a webpage or website is an acceptable source, and there's no easy checklist that will determine whether any one source is acceptable. It will often depend on your information need. For example, if you are analyzing online behavior on Reddit, then you might want to quote some posts from a particular Reddit thread that you are analyzing in your paper. However, if you are looking for reliable information about a particular company, organization, or concept, you would certainly not rely on Reddit posts for that information.
One helpful tool for evaluating websites is the SIFT method, which encourages you to search outward on the internet to learn more about a website before you read pages on that website. For example, it might make more sense to read the Wikipedia entry about a particular website to understand the larger context of the organization that runs it than to read the "About Us" page on that website, which will always frame the website in the most positive light possible.
Another great source evaluation tool is the ACT UP method, which helps in understanding bias and privilege in the information landscape.
If you have any questions about whether a particular source will be acceptable, you can always reach out to a librarian for advice, but you should also discuss it with your professor, who will have the best understanding of what is appropriate for the assignment, the subject matter, and the academic discipline. Different professors will likely have different sets of criteria for what sites can be used.
