, how do you know it is being aimed at you only when it is on the way up?
)
because they don;t fly straight up they travel fairly horizontally once they're going. Russia and America spent decades figuring out how to calculate the target from the launch phase (the current/past methods have probably never even been released). it's not precise but its a "it's coming in our direction, it could hit us if it comes back down" kind of thing so better safe than sorry.