Certified psychologist and an individual test like the Weschler or Stanford Binet. Even then, they have significant limitations. They can be STATISTICALLY accurate. Meaning they can compare the performance of people on one test and rank them, giving them a numerical score. But that's only a measure of what the test measures!
Some can be used as predictors of achievement or performance. Again, it depends on which test and what it measures.
Wikipedia has a good overview of the process. But if you absolutely need one, go to a certified person...
[This message edited by scaredyKat at 8:32 AM, September 5th (Thursday)]