The bell has two different rings, a high pitched and a low pitched version. Usually, the higher pitched one plays first (start break time), then the lower pitched plays second (end break time). Went and tested this after I posted the original comment. There's some difference...not exactly sure.
Could also estimate when you last heard the bell. If you haven't heard it in awhile, safe to assume it's break time. Higher difficulty slightly changes this...tougher estimations. Go with this option instead of the one mentioned above...