I added the question mark to the title because I'm not 100% sure this works, but it would seem to. As you know Google has the power to stop any page/site from ranking from any given single keyword or keyword phrase. Google does not warn you they've done this.
So how can you tell if you can no longer rank for a specific word?
Old way - look at your websites sitelinks and change the page title until you notice a word vanishes. This took time but gave away the fact you could not rank for that word. Google has since fixed this.
New way(?) - If you perform a search in Google for various keyword phrases and find your top ranked page you will see some words highlighted and some not highlighted. If BOTH your search query AND your page title have the same keyword but the serps do NOT highlight the word it means a)the word is a stop word or b)you can't rank for that term.
To be sure you can perform a search for a keyword phrase that includes that keyword and see if the word highlights for other sites. If the word highlights for others but not for you... you're not ranking for that word.
Note: Google seems to have fixed this when performing a site search, all matching words will be highlighted, but perform a regular search and (until Google fixes it) you can SEE which words Google isn't attributing to your site/page. Use this while it lasts to spot possible keyword de-ranking.