There are no objective tests for migraine; the diagnosis is on the basis of the history and pattern of symptoms over time as well as response to any drug treatments tried. This is why keeping a migraine diary can be helpful.
Things that should be covered in a migraine diary are:
How often you have headaches.
Where the pain occurs.
How long the headaches last.
When the headaches happen (such as during your period or due to other external stimuli).
Other symptoms that occur (such as nausea or blind spots).
Any genetic family history of migraine.
Any medicines you are taking, even the over-the-counter medicines & supplements.
Any medicines you have taken to treat your symptoms and how effective they were.