who's decision is it to determine this?
Ultimately, it will have to be determined by the courts. The implication, however, is that since openly carrying a rifle is legal, the cause for alarm must be inherently more than openly carrying a rifle- through actions and demeanor.
If the guy is muzzle sweeping,and running around like he's in a 3 gun competition- ok. But it would be difficult to suggest a guy walking at a lesiurely pace with it appropriately slung was trying to cause alarm.