The course logs might help you with this. In the course administration block, click on "Reports" and then "Logs." You can filter by student, activity, and date. Choose "All Days" for the date. Click on "Add" for the activity type so that you can view all of the posts that the student added to the forum. If you click on any of the logs, it will show you the post. The logs will be listed in chronological order from the time that they were made, so you can rate the first one if you'd like.
Alternatively, you can rate them all and take an average of the posts and make that the students' grade. This can be done on the settings page for the forum. In the ratings section, you can choose to take an average of ratings for the grade.
The thing that I don't like about ratings is that a student could meet the requirements and post high quality posts, but if he/she makes additional posts and they are graded then the grade on those can actually lower his overall grade. So, you might want to just rate the first post that's made and not the others. Or you could always go back to the manual grading method. Either way, referring to the logs should make things easier and more efficient for you as the teacher.