I think that a combination of written anonymous student evaluations with interviews of students are key. Also as a professor it is always pretty clear to me whether i have done a good job or not...so an objective self evaluation is critical as well.

I think that the three things that i am most concerned about when developing an online course for students is 1) their ability to understand what is required of them, 2) making sure that the course material is easily found and easily recognized and 3) that i have as much opportunity as possible to "interact" with them through a variety of media.