What determines you have learned a (theoretical) subject?
I asked myself this question a lot during my undergrad - "Did I learn anything here?"

This is how I determined if I have learned a subject:  Can I teach it?

Now, I don't mean can I get certified to teach it, or can I go teach the course in the next semester. If I could sit my parents down and explain to them subject X and have them grasp it then I believe I have learnt it. If you can't inform someone about a subject well enough that they understand it then you cannot possibly understand it. 

You could also write a (fake) course outline. Pretend that you would be teaching this subject as a course. Create an outline such as

1). Major point

   A). Sub point

      I). Explain sub point with case x

      II). Explain sub point with case y

B). Sub point


Etc.

This is what I do when I tried to determine if I learned enough about a subject and if I felt as though, for example, on main point 5 sub point B I can't explain it very well or I am even too confused by it to really grasp it - back to the textbook, or google.