Just do a search for your keyword or phrase and see how many results there are. Find one with a low amount of results and go for it.
I use Google web tool , put in the keyword and find little or no comp. and Avg serch results . good luck
Put in a search term such as "golf clubs" [ just an example ] at google using the quotation marks like I just did. If the return is over 6000, you will probably have plenty of competition for your chosen niche. If it has plenty of returns without the quotation marks it obviously has potential though, and with some extra work would be possible to successfully take over that niche if you have buyers that have money and are willing to part with it.