I've written a whole big blog post on this topic some weeks back, have a look at http://www.agicent.com/blog/how-to-validate-app-idea.

You can start with as simple as doing google search the idea and see how many competitors are already there, and if there is no one at all, and then can go doing as complex researches as doing competitor's analysis, using tools like keywords planner, vima, ideasquare etc, and also take help of your friends.