Hi Elizabeth:
Pretesting is just a tool that can help understand the general classs knowledge and assist the instructor in planning or making adjustments in order to place emphasis where it'll do the most good.
Besides written, another approach may be to compose a list of questions that contains some or all of the course objectives and related topics. Then the teacher could read a question to student A, then ask the class to give their opinions about the answer. After discussion, the teacher would go to the next question and next student and repeat the process.
Although more time consuming then a written test, between the questions asked and the discussion that followed, the teacher would have a pretty good idea where the weaker areas might be.
As always, in open discussion activities, the teacher has to careful that certain students don't dominate the discussion. Effort to draw in everyone is essential.
Regards, Barry