How can I tell if a liquid is (not) emitting light?
In a photograph, a light source is always brighter than any surface it is illuminating.

Therefore, if the liquid is the actual light source, the liquid should be significantly brighter than the rest of the bottle. See for example this picture by David Mülheims CC BY-SA 2.5, via Wikimedia Commons:



In the given picture, however, the parts above and below the water level have a similar brightness.