This very good video from CSUCB's Pfau Library will help explain the difference between scholarly and popular articles and how to tell which you are looking at.
You may be familiar with the distinction between primary and secondary sources in the humanities. There, a primary source is an account from someone who experienced the event - a first-person account. A secondary source is written by someone who was not there.
Likewise, a science primary source is written by someone who experienced the event first hand. Just, in this case, the event was the experiment or the gathering of data. A secondary source is written by someone who has read about the research, usually from the primary source.
So how can you tell if an article is primary or secondary?
Other things to look for include:
There are good reasons to use either primary or secondary sources. Secondary sources can catch you up on most of the important research in a specific field, as well as providing over-arching themes and trends in the research. Primary sources give you much more detail on the research, including raw data. A single source may illuminate differences or problems with trends in the larger research body.
In the end you will need to consider both your level of knowledge and background in a field, as well as the specific requirements of your assignment. You may find yourself reading both types of sources to get a deep, comprehensive understanding of your topic.