If you want top quality work, ask them for a list of topics they are passionate about and a list they have experience with. If they have a passion AND have experience for your topic of choice, you have someone worth getting to know.
Even then, however, it is most important to CLEARLY communicate your needs, down to the nitty gritty detail. 85% of all copywriting problems come from the client/company not communicating its desires and needs clearly enough for the writer.
As the owner a content provider company myself, I would recommend that you ask for clips to online work if you need SEO content, and article examples in the niches you need for more traditional printed work.
References are always good as well, if you feel the need to really cover yourself prior to hiring your chosen content provider.