Before, examination most of the universities, conducts a form fill up for the students who are going to take the examination and in that form the enrollment number is given. So if your university does the same, then you will get the enrollment number in the semester examination enrollment form.

If not, then I would recommend you to contact the college officials and get it discussed, they will guide you out and can also provide you the information from there data base as well.

I hope this answer helps. All the very best for your future endeavors!