1) Yes, I started having migraines around the age of ten, though I no longer have them on a regular basis.
2) I can tell because the headache starts setting in immediately after smelling the scent.
3) Gaultier2 by JPG gave me a godawful migraine that caused me to miss class the day after I tried it. Timbuktu by l'Artisan and Santal by Floris are mildly headachy. The latter 2 don't have a particular note that I recognize as headache-inducing but rather a certain "aura" that I identify as harsh and synthetic. I've experienced this with other fragrances but I can't think of names off the top of my head.
4) I'd consider my reaction to Gaultier2 as "migraine strength" (I'm a long-time sufferer of migraines), but since I've managed to avoid that fragrance, I can't say that it qualifies under the diagnostic criteria.
Hope this helps.