如何识别iPhone/iPad用户的itunes账户信息?

I am working on a sync solution for my app, and I would like to be able to find out if the user who has just downloaded my app would like to sync the app's data with the same app but hosted on another device (e.g., installed on both an iPhone and an iPad), so I can prompt the user to set up syncing. Is there a way to either (a) get the user's iTunes account information (the email address or some other unique identifier), or (b) identify if the user's iTunes account has already installed the app on another device?I am working on a sync solution for my app, and