There are three main ways to estimate what the search volume of a keyword is. Here are a few of the most common methods:
ASO tools (e.g. Mobile Action, SensorTower, Appcodes, TUNE) – these will provide you with a search volume score, which indicates the popularity of a keyword; TUNE will actually provide a real estimate of the number of searches per month. These tools compile data from several sources including their proprietary databases, the google keyword planner tool, store autofill results and trending keywords.
App Store Autofill Results – by typing a keyword into the search box you can see the most popular keywords that include that word, such as “free (games, music download, music”). For keywords that are popular enough in the app store, related keywords will also show at the top of a search done from an iPhone or iPad.
Google Keyword Planner Tool – this handy, years-old tool provides data on what people are searching on Google.com, which while not the same as app store searches, provides generally applicable search trends.
Google Trends – this tool allows you to see the popularity of a Google search trend by country or state. While keywords must have significant volume to show up in Google Trends, Trends can help visualize the popularity of a search trend over time better than the keyword planner tool can, even providing data by the hour.