knowledge of Google Trends:
Google Trends is one of the best and most versatile tools available for SEO. Google Trends is a public web facility of Google Inc., based on Google Search, that shows how often a particular search term is entered relative to the total search volume across various regions of the world, and in various languages.
To use this tool you do need a Google AdWords account. Once you have that setup log into Google Adwords, and this will be your Google AdWords home page. Once you're here, click on the tools and analysis button at the top. Then, click Keyword Planner.