Carefully keep track of your research before you write.  A research journal and citation management tool can be a big help.

Make sure you paraphrase or quote your sources correctly as you write. 

Double-check that you've correctly cited all of your sources after you write.  (Use your style guide!)

But, if you want to double-check that you've done it right once your paper is finished, there are some free plagiarism checkers available.  Keep in mind that some of these sites offer limited free services...you may have to subscribe to use all features.  Here are two examples:

DupliChecker

Quetext

This site reviews several others:  Top 10 free plagiarism detection tools for teachers. 

Please note:  The official plagiarism detection tool used by the University is Turnitin (read more below).  The University and the Library do not endorse the free checkers linked above and cannot guarantee that their results will be accepted by your professor. 

APUS subscribes to Turnitin, which is integrated into the classroom Assignments tool.  The library does not have access to this feature in the classroom, so please contact your instructor with questions.    Click here to learn more about Turnitin at APUS.

Turnitin: http://hznzdfl.cn/turnitin