If you leave speedgrader and go to the quiz itself, the moderating tab gives you access to the quiz log of each attempt. That log will let you know whether the student continued working beyond the answers that you see filled out. It shows you moment by moment which box they were typing in with time stamps. In my experience blank boxes mean the student stopped before they finished the quiz. If the quiz was timed and/or had availability dates programmed, it will have auto-submitted at whichever came first (the maximum quiz time or the until date/time) even if the student was not finished.
