Detect if iphone/ipad connected to VPN via public wifi programmatically

We have an app wherein certain functionality needs to work only when the user is connected to a router associated with his account (we have some information about user accounts and home routers). However, it is possible for the user to set up a VPN connection to his home router via any public wifi/3g network. We want to block certain functionality if that is the case.

Any ideas as to how it can be detected using any of the existing iphone/ipad apis?

Thanks, Hetal