Having done GCSEs a long time ago now, I find the main way of understanding whether or not you understand something is by explaining it to someone else. Get a parent or friend and explain to them what you have learned, if you struggle to get them to understand the topic you may need to spend more time revising said topic. The opposite is true as well if you can explain a topic or idea to someone and they understand it, you yourself have a clear understanding of the topic itself.
As someone who has now worked in an apprenticeship for 5 years, it genuinely is one of the easiest ways to measure your knowledge of a topic, and it is more often than not a barometer of how people in the corporate world would rate your knowledge on a subject area.
Reading around the topic also helps a lot as sometimes concepts are simplified depending on the level of education you're at. Getting a more wholistic view could help you understand the topic more.
Hope this helps.