One symptom of too-wide handlebars can be persistent pain behind your shoulders, and/or a feeling that your shoulderblades are always sort of squeezed together. Some pain is normal at the beginning of the season as your muscles adapt to the "new" position, but persistent and acute pain is not.

I have realized that my handlebars are too wide when my boyfriend commented on the unusual size of my deltoid muscles for a women. At my next ride, I noticed how I was actually using those muscles more than I should, and that my arms were extended wider than shoulder-width on my bike. After three months of this "discovery", I still haven't acted upon it, mostly because adaptation has made this painless, even on very long rides. But my next significant investment in my bike will probably be a complete re-fit and, if the problem is confirmed, new handlebars.