Every element of a system leaks all kinds of related information, when you think of why it must be that way, consider what sort of elements must exist to support it, what similarities they must have, etc. I look at something, just a small piece at a time, then explicitly think through everything I can tell and what I can guess from that small piece I've considered in isolation, look at a bit more, occasionally throw out a bad hypothesis as a clearer picture emerges, look at the deficiencies in what I've done and figure out what should exist to prevent or obviate it, and often find that yes, such a thing exists.
It's hard to explain, kind of a style I've developed on the job, where you really just sometimes need to know things yesterday. When I consult other resources, it clicks better because it takes the haziness out of my sometimes half developed ideas, but the work I put in beforehand makes me remember it close to as well as if I figured it all out myself.