Comment by throwrqX

Well like any subject, you start by studying their history. Declassified documents take a while to become declassified but are a rich source of history. Then historians write books based on those documents and you can get a more reasonable idea of how things worked and fit into a broader picture. For more recent activities it's obviously more difficult but occasionally you get leaks that give you a sneak peek of what is happening (eg Snowden).