"Is the glass half empty or half full?" is a common expression, a proverbial phrase, generally used rhetorically to indicate that a particular situation could be a cause for pessimism (half empty) or optimism (half full), or as a general litmus test to simply determine an individual's worldview. The purpose of the question is to demonstrate that the situation may be seen in different ways depending on one's point of view and that there may be opportunity in the situation as well as trouble.
Another perspective comes from psychology, where research has shown that a speaker's choice of frame can reflect their knowledge of the environment, and that listeners can be sensitive to this information.
