If you are thin and have a moderately large-sized AAA, you or your doctor may be able to feel it as a "beating heart" deep in the abdomen around your belly button.
Many are incidentally discovered as a result of medical imaging for other conditions, by ultrasound exams, CT scans, MRI's, or even plain X-rays of the abdomen.
If you are over 65 or over 55 with a relevant family history you should advise your doctor and have an ultrasound. It is safe, fast and painless.