The point of resumes is to see if the candidate actually has the requisite skills for the job. The point of interviews is to verify what is represented on the resume is likely to be true. And, as the op is suggesting, a small contract would verify that the candidate can complete projects for the company and to their satisfaction.

he may be farming it out to oDesk or Elance at a fraction of the price

This is just plain paranoia. No one would do this and expect to be employed at a company for very long. Also, if you've ever farmed work out to oDesk/Elance, you might not want that work to represent you in an interview process (even if you were able to have it completed in a reasonable amount of time).