Look for the author(s). What academic degrees do they have? Where do they work? Are their degrees and work experience related to the topic of the article? Could they be considered to be experts in the subject?
Trade sources are usually written by professionals in the field or by staff or freelance writers/journalists. These writers and journalists may have expertise in writing, editing, and journalism rather than the topic of the publication/article.
Look for the content and purpose. What is the article trying to achieve? For example, is it trying to entertain, to inform, to sell a product, to report on research, to convince, to argue, etc.? What is the intended audience of the article--the general public or experts in the topic?
Trade sources generally have the aim of educating people in a specific profession about a topic or development in their field. As such, they will often use technical terminology from the field without explanation.
Larger Publication/Review Process
Look for the title of the larger publication in which the article appeared. What kind of publication is it? Who is the publisher--a publishing company, a university, a professional organization? Does this publication have a peer review process?
Trade sources are generally published by professional organizations or publishing companies.
Trade sources undergo editing and review for things like style, grammar, and clarity.
Look for a bibliography or works cited list. Does the article cite other sources in a formal way?
Trade sources may mention other sources or quote experts, but will likely not have formal citations.
Look for images. Are they mostly artistic and decorative--for example, a photograph of the research lab? Or do they provide informational content necessary to understanding the article--for example, a chart that provides data from an experiment?
Trade sources generally have images that are mostly decorative or artistic. However, because they are written for professionals in the field, they may also have more technical diagrams, charts, etc.
Note: The answers to some of these questions may not be in the article itself. You may need to do some searching online to find the answers.