App rank checker uses Google custom search API to find the rank of your APP for a keyword. You can get rank for up to 5 keywords for free using this APP.
How to Use the APP
1. Install APP and click on settings icon.
2. Click get API key. You will be redirected to Google custom search API page. 
3. Create a new project and copy the API key.
4. Enter the API key in the settings page of the APP.
5. Enter your APP Id in the format "id=app.vionixstudio.com"
6. Select the location where you want the search to happen. By default, its US.
7. Enter the keywords for which you need the rank.
8. Press Get Rank. The rank will update the position if your APP is within the top 100 results.

Google custom search is limited to 100 queries a day for free. After that you need to setup billing with Google. Each keyword search uses 10 queries, so you will be able to do a maximum of 2 searches in a day.

You can check out the help page from the APP for more info.