Rather than give a pretest to students in a course where new students may be entering, I usually ask each new student to introduce themselves and whether they have any medical background. This way I have a better understanding of each student's background and can read their facial expressions as to their understanding of the subject matter in most cases. After I assign the in-class activity, I always ask each student (new and seasoned) 'what questions do you have?' as some students may not want to ask questions during the lecture.