Original research. These include experiments, surveys, observations, etc.
AND / OR
Published literature reviews on your topic. If someone has already done a literature on your topic, or a closely-related topic, you can see what articles they thought were important. Don't rely on these articles alone.
Look at the article title (highlighted pink in the screenshot)
Many times the article title itself will give you a hint as to its content. Most literature reviews have the word "review" in the title. However, in the second example above, the article title begins with the word "effects." How would the authors know what the effects are if they had not researched it? Thus, that article is likely original research.
Look at the subjects (highlighted purple in the screenshot)
The subject terms underneath the article title often say things like "research", "qualitative research", "literature reviews", etc. If you see mentions of statistics, methodology, surveys, comparisons, etc. - these subjects generally indicate that you've found an original research study. Literature reviews often have the subject "literature review"!
Look for tables and charts (highlighted yellow in the screenshot)
An article that contains tables, charts, and graphs is likely an original research study. These graphics generally outline parts of the methodology (usually tables) or they are analysis of the data (the charts and graphs).
Read the abstract
An article abstract is usually a one-paragraph summary of the article. You can often read the abstract from the library databases, just by clicking on the title:
If the database doesn't have the article abstract, you will often find the abstract at the beginning of the article itself.