The quality of a source becomes clearer in comparison with other sources of the same type.
For example, it is not clear what conclusions can be drawn from a journal article unless it is read in conjunction with other related papers.
There are many possible criteria for making comparisons. You can look for differences in argumentation. You can compare texts in terms of the formal quality control applied in their production. By comparing it will become clearer if you need to continue searching for more information.
- What is included and left out in the argumentation?
- How do scholarly paradigms colour the author’s presentation of circumstances and phenomena?