I actually may have that Spanish national insurance number, which is valid for life, from the time I studied in Spain. That was years ago and I don't remember if I had it, but I did some paperwork, did open a bank account in Spain (don't even remember in which bank), and did have health insurance as a student. Is there a way to find out if I have that number, and what it is?

As I understand it, you would have to go back to where you got it. They should be able to find it for you.

A good gestor should also be able to help.