1. What is it about? The title will be your first immediate clue. If available, read the abstract (a summary of the article). If there is no abstract, read the article introduction and scan the article headings. Consider how the item relates to your research question and how you might use it. More on evaluating source purpose
2. What is the subject area focus? Knowing what discipline an article comes from can help you decide if the article is relevant. For example, if you are researching global warming activism for a political science class, an article on global warming from a chemistry journal may not be helpful if it doesn't focus on political issues. Look at the title of the book/article or the journal title to try to determine the subject area.
3. Are you looking for recent information? If so, look carefully at publication dates.
4. What type of source is it? Consider what types of sources or information you need in order to answer your research question.
For example, sometimes you may be asked to use only scholarly sources.
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.