I feel ya, man.
The weird thing about AoS is that it seems to take a 'positive rights' approach to rulesets but, unfortunately, it is incredibly un-specific regarding what 'rights' we are given at times. In a way, I like that: for the most part, it allows a very nice, loose-ish sort of game between to friendly folks. At the same time, it creates unnecessary tedium and brief confusion with regards to rulings such as these. If I were playing it, I'd rule it in the ways of olde:
Can't tell whether you have sight?
1) Stoop down, look behind the model's head. Lower your sight 'til the top of the model's skull or helmet or whatever obscures the model in question.
2) Can you see a body part of the model (not counting wings, horns, weapons and other aesthetics)? Stuff connected to the torso - can you see that?
If so, it's in LOS then. That's how I'd do it, anyway.
Per the rules, it appears to be 'any point.' It's either 'any point' or no ruling at all. Your call. *shrug* Meh! That's beardy imo. "I can see the tip of the feather on your helmet, sooo.... that counts as 'a point' and I can shoot at it, right?" Weak!