What: Explicitly asking students to share with you why they are taking your course and what preparation they have (if applicable) for taking the course.
How: You can hand out notecards for students to write on, this can allow for more open ended answers. If you have specific questions you would benefit from knowing students' answers to, you might create a short questionnaire to hand out and collect on the first day of class.
Why: This exercise is helpful for the students to think about why they are taking the class, and be able to articulate it. For you, this can be helpful to see where each student is coming from, and what their reasons are for taking this course. This might be helpful later on in the semester when you are trying to balance the needs of all the students in the classroom.