Face: look at the person's face and ask them to smile. Has their face fallen on one side?
Arms: ask the person to raise both of their arms and keep them there. Are they unable to raise one arm?
Speech: ask the person to tell you their name, or say 'hello'. Is their speech slurred?
Time: if you spot any of these signs, always call 999.
Learn it. Share it. You could save a life.