Is learning always observable?

In my opinion, learning is observable, but cannot be clearly observed. It is because learning depends on the student's characteristics, thoughts and conditions of the day to determine whether or not it is observable. For instance, we learned there are two types of students: those who are extroverted and those who are introverted. What if a student is an introverted person and is feeling too embarrassed to share his/her own idea about a certain topic in front of others, even if it is one on one discussion? What if an extroverted type student is not in good condition and does not speak actively in front of classmates as usual on that day? Can teachers observe the student's real learning? I do not think they can show the teacher their best performance. Therefore, students' learning is not always observable or clear on how much students learn about the subject.
However, even if their learning is not clearly observable, there are many ways to be able to observe students' learning. For me, pair and group work and one on one discussion are attractive ways of observing students' learning since students who tend to be introverted feel more comfortable to discuss a topic one on one rather than speaking in front of class. On the other hand, students who tend to be extroverted also can speak more actively in the small group or pair work. Because of this, I got interested in my classmate's idea about students who are extroverted and introverted making pair and discuss. I think that it is an effective and a good idea.