What you need is a guided tour by someone who knows the database and can explain the general principles and the strengths and weaknesses. if you're lucky there is someone who has been around forever who can spend some time with you explaining the schema.
I tend to set up diagrams for any new databases I create as I think they give the best overview of the structure and you can annotate these easily enough.
A good understanding of the business area helps you get an idea as to how the tables might be structured. Sometimes if you try and suss out the key areas that the database deals with and work out in your head how you would set up the database then often chunks of the real database fall into place.
An understanding of the history of the business, db, applications etc also helps.