1. Obtrusive: demonstrations, projects, discussion
2. Unobtrusive: when the student is being assessed
3. Student Generated: shows students' levels
Based on classroom observations and assessments, give what you are used as data. How can you use that as a grade? Some students do understand content and can be evident in classroom observations and discussions. It often can be hidden on assessments due to different factors, such as language barriers, test anxiety, etc. There are many ways to measure the student's proficiency.
http://www.youtube.com/watch?v=mfOnyrWtPu0