There are three easy to spot signs of stroke. Since a stroke affects one wide of the body you can use these symptoms:
Face dropping. If it isn’t obvious you can ask the person to smile which will make it more visible if one side is different than the other.
Arm weakness. Ask the person to raise both hands above their head. Is one starting to drift down?
Speech difficulty. Ask the person to say a sentence like “the sky is blue”. Is speech slurred or hard to understand?
If any of these symptoms are present you should call emergency services.
Read more about stroke at the American Stroke Association website.