Analyze articles for keywords ?

Is there a way/tool to analyze articles for keywords being searched for ?

I know that most/all people have keywords where they write articles for, but i have quite a lot of articles where my main keywords are not relevant as i am in the process of making somewhat bigger websites and want a lot of content of which some is only far/not related of my main keywords.
Many of the articles are good written and i do not want to go and stuff them with keywords but instead use the words in the articles as keywords and rank for them to drive more traffic to the sites.

I know i could manually search the articles and use words to research but i wonder if there is a tool or easier way to let's say 'scrape' the articles for good keywords.

Hope i made any sense.

thanks

content composer tells you the keyword density of all words in article.

I think you should also check google`s wonder-wheel and the LSI keywords tool at lsikeywords.com . Good luck.

Don't get me wrong, I do understand how to get keywords and related keywords .

The thing is that i want to work backwards on some articles.
Normally i would find keywords/phrases and write or let someone else write articles for it and seo the article/page/site for those keywords/phrases. Usually i focus on 2-3 keywords/phrases per article.
I think everyone understands that.

What if I have articles of 1k-3k words and want to find the 'best keywords' besides those 2-3 keywords which i focus on (best keywords as in exact searches) from that article and then optimize.
Or have articles which are not optimized and i still want to put them online in some sort of usefull form to atract traffic.
The reason is that i noticed several times from my sites which were 'optimized' for a specific or several keywords/phrase that people where finding my articles/site through searches where i did not even think of or not related to the specific site at all.

example : i have an article about outdoor swimming pools and somewhere in the article there is the phrase ' i took of my wet tshirt and went back into the swimming pool. ' ( yes i wrote tshirt by mistake instead of t shirt in the article )
what i noticed is that i got hits of people who searched for 'wet tshirt' .
So what i am looking for is a tool that can discover these 'hidden' keywords in articles so i can optimize pages/articles for more keywords then i iniately have done or even when i have no specific keywords in mind .

thanks