After writing your article, it is really difficult to manually check the density of keywords. Let us take an example, your focus keyword is "WordPress Theme" and you have 1500 words lengthy article. In this case, you need to find how many times you have used "WordPress Themes" in your article. This might be an easy task to search. However, the difficult task is to find the related and other keywords you have written without your knowledge. Probably, you might have used "WordPress Hosting" more than "WordPress Themes" in your article. This makes it complex as the article's focus keyword is completely changing unintentionally.
Here is a free article keywords density checker tool to get all repeating keywords in your article. As explained, this will help you to make sure you are focusing on your keywords and find related keywords that you can still optimize the article.