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.
How can I be sure an article is peer-reviewed?
A quick way to tell if an article is peer-reviewed is to look for "submitted/revised/accepted" dates on the first page of the article. These tell you the dates the article went through the various steps of the peer-review process.
You can also look up the journal in UlrichsWeb. Peer-reviewed journals have a symbol that resembles a referee's jersey on them: Be sure to search by the name of the journal, not the title of the article.
Finally, the most authoritative place to look is the journal's homepage. Do a Google search for the name of the journal. Look for a description of the journal that says "peer-reviewed" on its homepage. Also, look at the information for journal editors, article authors, or review policy pages. These will often detail the peer-review process for each journal, including which sections of the journal are reviewed and which are merely edited.