Credibility is a contentious issue, and largely a matter of perspective and opinion; what one person may consider to be an authoritative source may be rejected by another. However, most academic authorities accept material from websites with domains ending in .gov or .edu, because these sites are controlled by regulated institutions that are responsible for all the material that they publish on the internet. In general, you can assume that material on these sites is credible, because it is (presumably) vetted by people who are held accountable for it.
There are a few criteria that any credible source should provide, such as;
- Authorship, citation of sources, and (possibly) an associated institution.
- Evidence that the author has some experience or accountability for the work (such as a published biography, contact information, or previously produced material)
- Credible sources will often describe how they acquired the data being presented, or will provide a means by which interested parties can acquire the complete data and procedure.
There are also "common sense" conditions that, while not a hard and fast rule, might indicate credibility, or lack of it. These include;
- Credible sources are most likely written in a professional tone, free of spelling and grammar errors.
- Credible sources aren't very likely to make outlandish claims, such as proof of a perpetual motion device.
- Credible sources are usually free of bias.
- Credible sources are almost always peer-reviewed. While a self-published or unreviewed source may indeed be credible in theory, in practice we would not consider it credible because it is too likely to reflect the author's bias or perpetuate errors.
Another means of checking for credibility is simply looking up the data presented in the source, and seeing if it is cited or referenced in other publications, or published on a variety of sites.
If it's information that can be easily changed (*cough cough* Wikipedia) it's not a very credible source. If it's an account of a historic event, it should be written or recorded around the time the event occurred. It should also not be biased (it's very easy to know if something is an opinion or biased). Common sense also plays a big role in knowing if something is a legitimate source. If something has grammatical errors and spelling errors DO NOT USE IT.
A credible cite will almost always have the date it was published somewhere (usually at the very bottom of the page) and sometimes renewal dates. This is a good means of determining whether the content is up to date and accurate, especially concerning time sensitive material.