"If you’re incompetent, you can’t know you’re incompetent …the skills you need to produce a right answer are exactly the skills you need to recognize what a right answer is."
In 1999 by David Dunning and Justin Kruger (Department of Psychology, Cornell University) published the results of a series of experiments looking at people's ability to judge their own abilities (.pdf). Subjects completed a 'test' and a self-assessment in the area of either logical reasoning skills, grammatical skills, or humor.
The first result they found was that those who were the least competent in these areas were the most likely to overestimate their abilities. In fact, most people overestimated their abilities, but those who were the least capable overestimated their abilities by the most. Those in the top quartile, conversely, were likely to underestimate their abilities (this belief that you are more inept than you actually are is sometimes called the imposter syndrome).
The second finding was that when those in the bottom and top quartiles were brought back in and asked to 'grade' the response sheets of other subjects, those in the bottom quartile did not adjust their belief in their own abilities by much, but those in the top quartile did recognize that they had done better than the other subjects, and correctly upped their perception of their ability.
The most striking result is perhaps the first one: low achievers that had test scores that put them, on average, in the 12th percentile estimated themselves to be in the 62nd percentile.
However, the second result is perhaps more important; it indicates that the ability to know how good you are at something depends on actually being good at something. Likewise, if you are not good at something, you are much less likely to recognize the cues that tell you that you need to improve -- and therefor, you won't.
While this effect has been confirmed in multiple areas -- including in academic subjects and social awareness -- and across various difficulty levels (i.e., even when given harder tasks, the inept still think they did well), most studies have been done in America. Studies of East Asians indicate that they are more likely to recognize personal failures, and are much more likely to work to fix these problems than are Americans; this suggests that the Dunning-Kruger effect may be strongly influenced by culture.
But even if you are American, there is hope. Dunning and Kruger also found that targeted tutoring was effective in both improving the performance of the lowest quartile, and in readjusting their perceptions of their abilities. So while you can't tell what training you might need, you can benefit from training when it is provided. Which is why you should never stop learning -- and perhaps, move to Japan.