I find that I must spend a significant amount of time on every new page of technical content to really understand it and soak it in. As I'm reading through any publication I stop after each page and mentally quiz myself on what I've just read just to be sure I'm not faking the funk. It's one thing to understand a concept as you're reading it but I would say that's not enough--you should be able to recall it from memory with the ability to explain it to someone else who has never been exposed to that concept.
This worked for me at my first few jobs and it's working for me now in learning complex systems and tactics as they relate to flying and employing military helicopters.
To augment my reading I also use the spaced-repetition app Anki. This has proved invaluable to me and I currently have over 2000 "flashcards" related to my current occupation.