Testing your Knowledge while studying is very important. It helps you to know how much you know about a Topic and how much more you need to learn.
Here are a few ways you assess your knowledge on a topic.
Take a quiz
Taking a quiz will help you see the areas you need to study more. It will open you to new branches of the subject. Find a quiz online related to the topic you are studying, take it and see the results to know where you are lacking. You can also ask someone to prepare and ask you some questions on the topic you are studying.
Do some general research on the topic
Research and find out if there are updates or branches of the subject that you don't know about. The internet, books or archives are good for doing your research.
Ask an expert in the domain
Find an expert on the topic and ask some questions. You may be surprised how much you know or still need to know. It could be a professional, a teacher or researcher.