There are no diagnostic tests that can determine if someone has migraines. Migraine is a primary headache disorder (no known cause). An MRI is usually done to rule out other causes for your headaches just in case. Most of the time, the scans are clear, so I wouldn't stress about it too much. If something does show up, the good news is that you have a secondary headache disorder instead (cause is known and probably fixable).
For example, one of my family members started getting terrible headaches suddenly. No medicine would ease her pain. Her doctor did an MRI to rule out structural problems and found a tiny benign tumor at the base of her skull. It was putting pressure on nerves and causing her headaches. After surgery, her headaches went away and she had no more problems.
Her story is rare though. Expect this to be a routine test that finds nothing unusual. Once the fixable stuff is ruled out, then you know for sure it's migraine and you can get on with starting a good treatment plan to manage them.
Here are links to a couple of articles that might help you understand the process a little more:
https://migraine.com/migraine-diagnosis/mri/
https://migraine.com/blog/migraine-testing-what-to-expect-from-an-mri/
https://migraine.com/pro/clinical-features-migraine-diagnostic-testing/
https://migraine.com/blog/what-is-the-difference-between-a-primary-and-secondary-headache/
Best of luck to you. Please write back and let us know how it all turns out.