MIDI Aid is a diagnostic tool for iPhone, iPad, and even Mac. The iOS version is Universal, and both it and the Mac app are $1.
MIDI Aid iTunes Description:
Having trouble with your MIDI controller or want to learn how it works?
MIDI Aid receives messages from external and internal MIDI devices and displays them in real-time in an easy to understand, human readable form.
It automatically detects any changes of your MIDI subsystem, e.g. when plugging in a new device, and will immediately start displaying its messages.
MIDI Aid is also available for the Mac.
This app is ad-free and it will always stay like that!
Want to see how it works? Check out this video on youtube: http://youtu.be/Eyqntqykv4Y and leave a comment (feature requests, questions, remarks, etc.)
I've raised objections to people using Facebook as their app's official iTunes "support" link before, but this one takes the cake, linking over to a YouTube video! You're actually supposed to leave comments on the video for support inquiries. Here's the iPad demo. Crank it! No really, it's super-quiet, you'll have to crank it up.