Read the BDD style acceptance tests that were written during development.
This is the one chance you get to preserve that knowledge when it fresh and correct. Asking someone three years later (even the original dev) is no substitute.
Oh, and you can also run the tests to ensure correctnes.
You should know, that by removing all alcohol, caffeine and sugar, red meat, bacon, eggs and sausages from your diet, you can get rid of what joy you have left in your life.
We will learn this the hard way.