You can use tools like Long Tail Pro/Platinum (it analyzes the top ten search results and calculates "keyword difficulty" based on that), Market Samurai (it has a lot of features that help you get an insight into the competition and the CPC for keywords), and there is also an improved Keywords Explorer tool on ahrefs. All of those are paid options, but you can make a good use of a trial period.
I guess everyone has their own favorite tool, but you have to be aware that there is no tool in the world that is just going to spit the "low competition, high value" keyword for you just like that. It takes some serious research. Or luck!