DOI—Digital Object Identifier
If your APA reference has a DOI (digital object identifier), then use the DOI. It looks like this: doi:10.1111/1467-8535.00078. The DOI is a unique and permanent identifier that will take the reader directly to a document regardless of its location on the Internet.
Sample reference with a DOI:
Davis, B., & Simmt, E. (2006). Mathematics-for-teaching: An ongoing investigation of the mathematics that teachers (need to) know. Educational Studies in Mathematics, 61(3), 293–319. doi:10.1007/s10649-006-2372-4
To locate a DOI, plug in author name and title here: http://www.crossref.org/guestquery/
If a DOI exists (it won’t always, so don’t worry if you don’t find one), it will show up with a corresponding URL. Check the URL—if it’s broken, consider not using it in your reference listing; if it doesn’t work, it won’t benefit the reader.