The reliability (scientific nature) of sources can be verified by three kinds of checks:
- Check by others, before publication
- editors: editors of scientific journals are stricter than editors of non-scientific journals
- publisher: some publishers only publish scientific books
- peer review: some journals but also some book publishers ask experts for a (blind) judgment before publication
- search engine/online bibliography: some search engines only include articles from high-quality peer reviewed journals (for instance Scopus and Web of Science)
- financier: some journals demand that the names are published of those who have funded the research
- Check by others, after publication
- reviews (in the case of books): is the book review positive?
- citations (particularly in the case of articles:): how many times is the article cited and especially; what is said about the article?
- Your own check:
- who is the author and when was the article published (especially with web pages)
- affiliation of the author: the job may tell you more, for instance if the author is employed by a (good) university
- what is the intended audience of the publication (for websites and reports)
- how explicit is the phrasing of the question? Does the article contain conclusions?
- is the used method explained: how was the research organised, where do the data come from?
- are there enough references? Are they of high quality?: on which insights is the theory based?
- language use: level and grammaticality
In the special LibGuide Evaluating sources 
you will learn how to deal with these matters
