What symptoms will help a doctor diagnose brain aneurysm?

Thunderclap headache: Very abrupt onset within a few seconds of a very very severe headache (so called "thunderclap headache") is the most suggestive sign since this is how bleeding into the subarachnoid space (in effect bleeding from annaneurysm) within the cranium announces itself.

Headache: Or visual symptoms. Family history.

Many: An ususual, severe headache unlike anything you've experienced before would suggest a rupture, and a ct scan is the way to diagnose this. Otherwise, if the aneurysm enlarges sufficiently, it may compress other brain structures and cause a variety of symptoms. A cta or mra would be the test to further evaluate those symptoms.