The only thing that would guarantee you really understood something, is to explain it to someone else, and you'll have so many benefits.
Albert Einstein said: 'If you can't explain it to a six-year-old, you probably don't understand it yourself. ' For simplicity is genius.
So the first thing you could do to check your own knowledge, is to try and explain it for someone else.
That is for sure not the only benefit, because every time you explain something to anyone, you find simpler ways to do so, and you make new maps for this knowledge in your mind, like adding the new things you have learnt to the old ones.
Another benefit is, and this one I've personally experienced, is that the only thing I don't ever forget, is something I explained for someone else.
God gives us rewards for sharing knowledge, so that we could help people and also help ourselves, knowledge lets people grow, and it could change their life drastically, and it will do so for you also.
For that reasons, never hesitate to explain something that you understand for other people, not only because they will benefit from it, but because also you'll have great rewards for doing this.
That's all for today, and I hope this was helpful, I wish you a happy great life, and wish I could see you here more often :)