Santa isn't omniscient. He sees you when you're sleeping. He knows when you're awake. We can infer from this phrasing that he can't see you when you're awake. He also knows if you've been bad or good but that information isn't as specific.
I think we're discussing the limitations of a fictional being. In the context of "Santa Claus is Coming to Town", I'm not sure his powers apply to the internet, being that "He sees you when you're sleeping...", but he only "... knows when you're awake." So if my incorporeal consciousness was uploaded to the internet, do you think I could spoof my online status to green, so that I'd be untraceable to Santa?
I reckon he knows if you've been bad or good because of the parents' assessment of their children's goodness. That is not a specific Santa power, except in as much as the power of Santa to get that parental assessment is a power
I think it has to be based on the parents, as clearly bad kids still get presents, and only their mother thinks they're good.