How to selectively and securely enable hidden test features in iOS application?

Question

I would like my iOS application to provide a special set of commands for testing only. I would like these commands to be available only to authorized testers, not to general users.

The iOS application has external connectivity to a secure web site I control, and I can also freely manipulate the iPhones it runs on in test.

How could I decide (programmatically and securely, at startup) whether my app should enable its test mode?