Some people say it must be somewhere at 2-3% - some say it`s not a ranking factor. There is no clear statement about exact numbers. To give it a definition: Keyword density is the measurement how often your keyword appears in your content. If the same keyword is repeated in every second sentence you easily have a density of 10-20% which is a clear sign of overoptimisation and keyword stuffing. Don`t spend time on keyword density and focus on creating quality content written for real people. Searchengines take care of the rest by checking related keywords and matching patterns (LSI) to find your article's topic.
Though it's overrated to focus on one keyword it sometimes makes sense to check your competitors density to know what works. But again - don't spend too much time with that. Write for people not spiders! Build your site around a niche - not a keyword. Google will know what your article is about.
It helps to mentioned your main keyword in the first paragraph to keep the flow between the title, headlines and content. (proximity density).
ANYWAY! WHERE CAN I CHECK KEYWORD DENSITY?
If you still want to check it here are a few tools: