3) Google: This is pretty straight forward but it’s the main reason I like it. I search for my main seed keyword in Google, and use the keywords that Google itself highlights in bold on the search results, plus the “Searches related to” section at the bottom to get keyword variations or LSI. That’s basically what Google is telling you that topic is about. No need for a thousands other tools. I use these to optimize the on page of my target pages as well.
However, this does not mean you cannot topple them. It just takes more of an effort in terms of content as your page has to build the trust. That is why you will see the "Google dance" happening for fresh content from a site that is not yet trusted or is not very authoritative. Google gives your page a chance and measures user click-throughs when it pushes you to certain spots in the SERPs and then measures user engagement levels when the traffic hit your site through those positions in the SERPs.