Answer and Explanation:
It really depends on what type of source you're using. Most book sources are at least somewhat credible but who uses books these days?! With online sources, usually sources ending in .gov or .edu are credible. .web I believe is usually rubbish and will be discredited by your professor (if you're in college) but not sure about middle school and high school teachers these days...
That's my main tip, it's been a long time since I took college English but the professor gave a whole lecture about credible and non-credible sources and that's like the only thing I remember. lol
Hope this helps :)