I developed an application with compatibility to SDK level 10.
In order to make the application available for wider range of users, I’m considering lowering the API level to 8.
Is it as simple as changing the minSdkVersion value in manifest?
How can I make sure that I didn’t use any API calls that exist only on API>=10?
Your app will run on previous platforms, Only if you did not use parts of API that are not available at previous platforms. One thing to do is to try to compile against the lower platform (You can install lower platforms in you SDK manager). The compile errors, if any, will point out the places in your code where you used non existing API. Also, watch out for deprecated warnings while compiling on recent platforms.