If you practice a lot of the reading and listening tests, you should have a good idea of what band you belong in and where your weaknesses are.
For Speaking and Writing - we used the paid tutorial site, and found their lessons quite helpful.
Even if you are a native English speaker, I would suggest a lot of practice - we spent close to 40 - 60 hours practicing before we attempted the test. During the Writing test you really need to pay attention to the time as it goes quick, and for the speaking - you need to be able to use formal language, and speak continuously on random topics for at least 3 minutes without repeating...so practice with a friend.
If you are looking for a computer-based IELTS mock test, then try the 8bands.com website. It provides one free mock test as a real test for demo and practice purposes. There are also many practice tests & mock tests available at a very low price.
Try fourmodules.com. They grade essays as well. But use it as practice material and don't worry much if they give a low grade. It is good for practising and getting used to the format and sitting for 3 hours straight!