I tried to make the best poll options I could (without making three questions with ten answers each or something), but basically, I:
yell "fore" when I can't see where the ball is going (i.e. it's over a line of trees or over a hill toward another tee box, green, etc.).
yell "fore" when the ball is likely to hit someone that I can see.
do not yell "fore" if I know that the ball is not going to hit anyone.
When someone else yells "fore," I:
cover my head and duck in place when I don't know the source of the yell or I can't see the source.
run away while cove