In reference to the question, "How can we tell that the milky way is a spiral galaxy?"
The answers there clearly sum up the question asked. But Milky Way is not just a spiral galaxy. It is further classified as a barred spiral galaxy.
Question: Which particular feature in the distribution of stars, or in general a feature in observations led us to believe that it is a barred galaxy?
Note: The edge-on picture is not sufficient to establish it, since a dense distribution at smaller radii could also arise from a non-uniform density on the spiral disk if modeled to fit the observations. We don't have any data from other angles or orientations.