Skimming through a math paper with a group
My answer may not be completely relevant but I still thought it was worth putting in. I have had a few such sessions and I realized a few things. Not all of them could be true in general and I might have been a bit extreme with what I treat as knowledge. Here goes:

Your level of understanding is directly proportional to a few things:


Your ability to frame and ask grammatically correct sentences as an individual. This also encompasses communication of ideas/questions to experts of the field. For instance, suppose you are learning Linear Algebra in a group. You have a few gaps which are filled by others in the group. However, unless you can form sentences using Linear Algebra "handles", its unlikely you'll get far in research. Literature is way to dense in keywords. Unless you can talk in terms of Column Space, Rank and Eigenvectors (Rather than Linear Combo of all column or the vector which only scales) you are far from knowledgeable.
Your ability to participate in discussions. Its not difficult to lag and be left behind in a group of impatient, overachieving academics. Further, it can be a bit demotivating at times when the senior students who read the same content (owing to their heightened intuition) seem to grasp more. As junior students, it is often necessary to substitute the lack of intuition by more work. 
Your ability to appreciate the nuances of the field. I cannot stress this enough from my experience. If you cannot appreciate the subtleties yourself, you didn't learn much. It doesn't take much to learn Elementary Fluid Mechanics per se. But, IMO, you actually "learn" when you go OMG when you see the transport equation and play with it till you are satisfied. An extension of this is motivation. When some concept is developed by an author, he doesn't write in random order. One of the most important aspects is to be able to understand why something is being developed. For instance, most Aerodynamics books start off with Euler Angles and then move on to Quaternions. Its simple to understand what Quaternions do and how to solve equations based on them. However, unless you know that they are used to prevent gimbal locks, there is no point of knowing about them.
The usual: Your ability to write alternate proofs, write codes (if possible), interpret the results of these codes and the usual other-than-textbook stuff.


One ending comment : Your level of knowledge is a function of how good you are in the group and how good you are without them. If you are able to develop proofs in those 3 hours together but aren't able to get started on your own later, you need to investigate whats wrong. 

If you want to know how good these group sessions are, find out how good you are getting at that field as an individual