Obtain a copy of the music the student will play during his observation so you can follow along with the performance.
Listen as the student performs the piece. Follow along in the music. When the student plays something incorrectly, such as a wrong note, a missed note or an incorrect tempo or volume, make a mark on the music.
Add up the various mistakes the student made. Write this on the top of the sheet music.
Discuss the performance with the student after he is finished to give you an idea of the student's perception about the performance. Ask questions such as "How did that feel to you?" and "How do you think you performed?" Keep track of the student's answers.
Write a short observation report. Include the number of mistakes the student made and write up how the student felt about the performance. Include other information, such as what you've noticed about the student's study skills, his general attitude in lessons and how he has improved in his violin skills. Include this report in his file and continue updating the file with reports every few months. Share the report with the student, his parents and/or other teachers.