First try using the standard methods to limit cheating (question pools, timers, presenting the questions one at a time, limiting feedback, randomizing questions, using as many subjective questions as possible, etc.) Grijalva, T., Nowell, C., & Kerkvliet, J. (2006). Academic honesty and online courses. College Student Journal, 40(1), 180-185.  Sadly, I'm afraid that we might assume it is more prevalent than we would hope.  Stuber-McEwen, D., Wiseley, P., & Hoggatt, S. (2009). Point, click, and cheat: Frequency and type of academic dishonesty in the virtual classroom. Online Journal of Distance Learning Administration, 12(3), 1-10.