How would you want people to find YOUR app? You must have thought of some keywords or phrases that when googled, should bring up your app. Try using those and see what occupies the territory as of now.

Another approach is to discuss your idea with as many people as possible. Maybe you haven't heard of such an app but someone else has. This should be most fruitful if you do this with your target audience. Even if there is such an app out there but your target audience hasn't heard of it, then that's a good enough reason to try building it.