Somehow this behavior got changed in moodle 1.8 (I haven't tested 1.7 yet), in moodle 1.5.x it used to be after you grade the essay questions, when you see the "results" tab in the "Action" field, if you have graded a particular student, it will show in green color, but if you haven't graded the student, it will show in red color. Please see screenshot.
Here is how you repo:
1. Login as admin or teacher, enter a course and create a quiz with "essay" type of questions
2. Login as couple of students, and type something and then submit your answers
3. Login as admin or teacher again, then go to the Quizzes link and then click on that particular quiz
4. You will see under the "Reults" tab, you can't tell which student has been graded and which have NOT yet been graded...
I have also proposed a patch here (see diff file), hope this helps.
This issue become very important especially when you have a LARGE class (e.g. 1000+ students) where a instructor has to track which students has been graded and which has not...