If you had a brain aneurysm how would you know?

May not know. Often asymptomatic. Can cause severe headache, visual changes, stiff neck, and other focal neurological abnormalities if the aneurysm is large enough or ruptures.
MRA or CTA. Unless you have neurologic symptoms that may lead your neurologist or neurosurgeon to order an mra or cta or cerebral angiogram you would not know. So a high index of suspicion on the part of your neurologist to order the studies is the only way to know before rupture of the aneurysm.