This article might interest you. It discusses some of the anatomy, functions, and neuropsychological tests related to the prefrontal cortex (PFC).
The prefrontal cortex (PFC) is a relatively big area and serves many functions, so there's not going to be a single test that evaluates PFC function. Some tests will look at inhibition (the ability to avoid an automatic or trained behavior), task switching (like the WCST mentioned above), attention (PFC is thought to be important for controlling directed attention), working memory (memory of a small set of items for a short period of time). Self control is a really important function served by the PFC, especially in modern society. Remember that everyone is different, so someone will a lack of inhibition might not necessarily fall into the category of having a brain disorder. A neuropsychologist will do their best to use the limited tools available to categorize you based on some generally agreed upon diagnostic criteria, and if you fall into a gray area then that diagnosis might not mean much.
As far as how to cure an abnormal PFC? If you are under 25 years old or so, time will help. Many people's PFC is not fully developed until their mid 20s. Cognitive training is unproven, but the human brain is really good at learning and changing. It seems that people can learn maladaptive behavioral patterns over time, so why can't they train themselves toward better patterns? If you want to improve your self control (say to stick with a diet), then use every opportunity you get to train your self control. Make it like a game and give yourself a pat on the back when you do well. If you really stick to it, your brain could eventually learn to prioritize this type of behavior and get better at it. Note that this type of training hasn't necessarily been scientifically proven, but it makes sense and it sure hasn't been disproven either.
P.S. I'd recommend that you skip any natural medicines that might be advertised as improving cognition. Best bet nutritionally is to eat well and drink a lot of water.