For a source to be considered reliable, it must reflect accurate historical information, meaning that sources that are written in a subjective manner may still be considered reliable, since all facts it includes are historically accurate.
Determining the reliability of a source also entails considering the following questions:
- Is there a consistency between the source and the available data relevant to the topic?
- Does is the source scholarly credible? This includes considering where and how it was published.
- Are there other sources that could validate the information given from this particular source?
Regarding usefulness, sources can generally be considered useful, irrespective of whether they are reliable. To establish whether a source is useful, you may wish to consider the following:
- Is the source relevant to what is being asked?
- Is the source reliable in providing the information that is required to respond to the question?
- Has the source provided insight into the question being asked?
I hope this helps!