One way to determine if you understand a domain or a topic is that you can create an organizational structure for the information.
If the organization is complicated or convoluted it is an indication that you do not have a complete understanding.
If the organization is straightforward and new information fits into the organization structure easily it is an indication you have a good understanding.
Focus on creating the organization and continually refine until you have a straightforward structure for all information on the topic or domain.