The first step of developing an assessment plan, and the first step of instructional design, in general, is getting a firm grasp of the actual instructional goal.  If you're lucky you will have the opportunity to do a full needs analysis with resources and access to the learners, chances are you will need to be a bit more scrappy and try and muddle through to dig deeper.  But it is a critical step.  I tend to try and keep asking, sort of a "Five Why's" approach when I'm in a pickle with someone who isn't clear or doesn't want to share.  I'm also not easily embarrassed so I don't mind acting a bit more confused than I am at times.  What are the methods that you might use to dig deeper when someone says they want a course but is less clear on the outcome they are looking for?