You can determine the relevancy of search results by asking these questions:
- Does the source answer your central research question or your minor related questions?
- Does the source answer the full question or just one aspect of it?
- How strong are the similarities between the questions raised in the source found and those in your paper/thesis?
- How strong are the similarities between the units of research and analysis in the source found and those of your own research. The unit of research may be a (historical) period, a person, a group, an area or country, a natural phenomenon, a process etc..
- Is the context of the research unit in the source the same as in your own research/paper?
- When has the source been published and when has the research on which the piece is based been carried out?
You have to realise that only seldom you will find a source exactly matching all your own questions. You will have to combine information from many sources to answer your questions.
The special LibGuide evaluating sources has more on how to evaluate information.