Mainly just books and journal articles that meet certain criteria are considered scholarly. Scholary sources are written by scholars. A scholar is an expert who has extensive knowledge of a particular subject. Scholarly works will reference other scholarly work, adding to the scholarly conversation.
How do you know if the authors are qualified scholars, researchers, professionals, or experts?
Check their credentials, and investigate what the credentials represent. Are they relevant to the subject/source?
Check their employment. Is it a university or government agency? Consider any bias associated with the employer.