For scholarly books, look at the publisher. (Is it a university press or other scholarly press? Do they describe their editorial process? You may need to Google the publisher to figure it out.) For scholarly articles, look at the title of the journal (not the article title). Learn more about determining if an article is scholarly.
Books and articles: Articles tend to focus on a very specific issue or analysis, while books usually address a broader topic. (Note, however, that some books consist of a series of article chapters.) Often the record in a library database will indicate the item type, but you can also tell from the citation.
Research studies: This may only be relevant in courses which require that a specific type of research be used (quantitative, qualitative, experimental, systematic review, etc.). The abstract usually contains clues about the type of study. Most research studies also have a "Methods" section that describes how the research was conducted.
Looking at the information about a source that you see on the screen (in the database record) can also help with evaluating the source's relevance. Elements to examine include: the article title; journal or book title (this information is often labeled "source" in the database record); publication date; subjects; and often an abstract (article summary).