Question

What are signs that a source might not be credible?

Answer 1

There are many signs that reveal a source might not be credible. First of all, you need to check if has an author (it might be physical people or an association, company etc). Then, have a look at the information, it is well-developed, organized and complete. Check writing style, if ideas are coherent and if there are grammar or vocabulary mistakes. 
- Date: make sure it is included not only the creation date but if there is any update or new material.
- Domain: if it is a .com. org web page. 

Answer 2

If the author is anonymous the source might not be credible. Websites that start with gov or edu are sources that are credible.