A Stroke is also called a Cerebro-Vascular Accident, or C.V.A. It happens when a blot clot that has formed somewhere in the body (usually in the legs) travels to the brain and clogs an artery, causing the tissue around it to die from oxygen loss.
A stroke usually happens suddenly and sometimes has little to no visible symptoms.
It is said that once someone has a stroke, there is a 3 hour window where, if medical attention is received within that time period, then the negative effects of a stroke can usually be completely reversed. But how can you get someone medical attention if you don't realize that they've had a stroke? Sometimes someone will fall down because they've had a stroke, but will seem to be otherwise fine.
So how can you check to see if someone has had a stroke? The easiest way is to remember the first three letters of the word "stroke": S.T.R.
The "S" stands for SMILE. Ask the person to smile.
The "T" stands for TALK. Ask the person to say a simple but coherent sentence, such as "It is very sunny today."
The "R" stands for RAISE. Ask the person to RAISE BOTH ARMS.
Another technique is to ask the person to stick out their tongue. If the tongue looks crooked or is not centered in the mouth, then they may have had a stroke, and paramedics must be called immediately.
All these seemingly simple tasks require the brain to send signals to different small muscles in the face & mouth, and require larger coordination in the arms. A person whose brain has been affected by a stroke will find these movements difficult to perform correctly.
Moreover, if you forget these techniques but are worried that someone has just had a stroke, simply ask them to perform any movements that require a little bit of hand-eye coordination. That would be better than nothing. So be on the lookout!