1. Check the admit card. The admit card issued by the Central Board of Secondary Education (CBSE) for the class 12 board exams will have the student's roll number printed on it.
2. Check the result. The CBSE class 12 board exam result will also have the student's roll number printed on it.
3. Contact the school. The student's school will also have a record of their roll number.
Please note that the roll number is a unique identification number assigned to each student by the CBSE. It is important to have this number for various purposes, such as checking results, applying for higher education, and so on.