"What test do I need to take to prevent a stroke or the test I already took give an indication"
Carotid arteries can be a source of disease, which can break off and cause a stroke. They can also restrict blood flow to the brain if diseased, causing fainting etc. The only way to see the condition of your blood vessels in the brain is a scan, such as a ct angio. However, I very much doubt if this procedure will be carried out unless you have symptoms to warrant it.
They checked to see if your carotid arteries were developing plaque...usually done during a cardiovascular work up...strokes are caused by many things..high blood pressure, thinning of cerebral arteries, congenital deformities to name but a few... google basis for strokes and learn more.