Unlike heart attacks, Ischemic (blood clot) strokes are typically not painful. Therefore the difficulty is that the only way to know that someone is having a stroke is to know the most common symptoms of stroke and if these symptoms appear suddenly, to suspect stroke and call an ambulance. These could include sudden facial drooping, sudden arm weakness, sudden slurred speech or having difficulty coming up with words.
Symptoms that appear over a longer period of time are typically not caused by stroke, so the sudden appearance of the symptoms is a strong indicator of a possible stroke. Remember in most cases the artery is blocked suddenly and the brain cell death starts quickly, therefore the symptoms also typically appear suddenly.