I've thought about this some, and I've figured that it is random chance that helps a combatant find the animating force. That random chance is determined by the need to roll 19-20. Initially, striking the animating force has nothing to do with skill.
Thus, I would say that, even when using magic missile, the attacker would still have to roll d20 in order for the MMs to hit the animating force. On any other roll, the MMs just harmless slam into the corpse and inflict no real harm--after all, the target is the zombie. The MMs autocatically hit the target.
Once the animating force is discovered, though, the caster can single out that body part. THEN, any further MMs will automatically hit the animating force.
How's that?