I have read that an indicator of whether we understand a concept or not is how well we can explain it to someone else: I would argue, then, that if we can't explain a concept we don't fully understand it. There have been times when I've said, simply, "I don't know." when I'm asked about a concepts, and, that's okay. I think it's good to run into information we can't transfer because it shows us an area we haven't learned as well as we thought. The solution, I think, is to study. There are ways to do this quickly.
Write Definitions: The practice of writing a concept in my own words, even if I don't keep what I've written, assists me when I'm put in a situation where I have to say it.
Explaining a concept to another tutor, even if it's "hey, can you tell me if I'm explaining this clearly?", can allow them to add their knowledge to ours, filling in gaps where information is lacking.
Team Tutoring: I've team-tutored with William a few times, and he's an expert (or well on his way to becoming one) on rhetoric. He teaches rhetoric so clearly and simply that I cannot misunderstand. I've since used what he's taught to help others understand.
I'll wrap this up with a story:
My dad was an Electrical Engineer and an expert on the Plumbing Code. Guys in his office would come in and ask for help about the Code: Everyone knew he knew it. My dad didn't need to memorize anything, he just knew where to find it. He pulled out his manual and turned to the right page, and that's the key: If we know where to find something, it can be just as good as knowing it because we can reference it again and again.