The answer to this question has to do with control over the work completed. Common traits of an independent contractor include: having other customers besides your business, having his or her own tools or equipment not paid for by you, operating under a business name, having employees, and advertising his or her services. Employees, on the other hand, usually have one employer who has a say over how the person completes work tasks.
If you are unsure, you also have the option of filling out a form and submitting it to the IRS for them to consider the details of your situation and make a determination. This may sound like a lot of work; however, if you repeatedly use independent contractors in the same role in your business, this effort may be well worth it. Download the form at http://www.irs.gov/pub/irs-pdf/fss8.pdf.