Are you thinking about taking a Cambridge exam? You can use Test your English as a free online resource to find out which Cambridge exam is closest to your current level.
After you've done this test, you can buy books with "past papers" (previous exams) to further check how you would score and what areas you need to work on. Of course, it's always good to consult with a reliable English instructor to find out which exam is right for you.
P.S. If you read this blog with regularity, you know that although I support taking exams if you feel you need to or want to, I believe that the most important thing in English is always communication!