PhanxBuffs did, for a while, have code in place to support showing fake/invisible buffs, and used it to show icons for monk and warrior stances. However, once monk and warrior stances were removed, I also removed the supporting code, since it didn't have any other use.
Based on the
comments on Wowhead it's "day" from 6 AM - 6 PM and "night" from 6 PM - 6 AM, but also "day" inside instances at all times. That (assuming it's correct) doesn't require any scanning of anything (and I'm not sure what you'd be scanning anyway, since there is no buff and the character stats API doesn't tell you where your stats came from), just a straightforward check:
Code:
if IsInIstance() or GetGameTime() - 6 < 12 then
-- it's day
else
-- it's night
end
If you use the default minimap, you can just look at the calendar button -- if it shows a moon and you're not in an instance, you have the "night" buff, and in all other cases you have the "day" buff.
Otherwise, it would be trivial to make a DataBroker plugin to show you this info, but since you can only ever get the "night" buff outside of instances (which seems like a good thing, so it doesn't affect gear tuning for raid content) it doesn't seem like it's really important enough to dedicate any screen space on showing.