App Store Market Researcher (ASMR) is market research software that provides actionable intelligence about what mobile and desktop games and apps are out there.
More importantly, you'll learn what users think of them. This information comes from the users themselves - people who install and use the apps, and have taken the time to share their views with the rest of us.
Your target market analysis will include an estimate of the market size based on how many apps have already been installed, plus potential market share based on how users feel about those apps.
Your search starts with the keywords for the apps you care about.