Popular Style Guides
Journal Publisher Style Guides and Manuals
Each journal will usually have a section on its website labeled "Guide for Authors," "Information for Authors," or "Submitting an article." This is were you can usually find what citation style the journal uses and see examples of at least a book or journal article reference. Some journals no longer make this information available but instead, refer the potential author to their preferred citation management software (ex., RefWorks, EndNote, Mendeley, Zotero, etc.) where their journal template is available.