Any more information about how and where I'd go about this?

But it depends where exactly in Europe you live. For example if you live in Milan, you can check it here: https://www.deltamedica.net/en/laboratorio-analisi/tests-and... - this lab is on the expensive side as they ask for €14.

Where I'm from, it's done at birth, it's just a standard part of a medical registration package thing your parents get. My parents kept my bloodtype card right next to my childhood vaccination records.

It's not like your bloodtype changes over time...

It can actually change in some cases. Liver transplants and bone marrow donations can trigger a change. Apparently kids a few weeks old can flip on their own as well.