I'm sorry but right now it's not possible to know when an AlarmClock fires, and could happen that some user could use a 3rd application which could make it more hard to communicate with every possible application since don't exists a standard-way (or i don't know if exists, i can't find after research).
The class AlarmClock
don't provide a way to manage it (Alarm Clocks will use AlarmManager to make this.) just a way to ask to the clock application to set an alarm.
Personally if you really need to do something like this you could work directly with the AlarmManager
class (which gives you more freedom on what you want to do, when do it etc.)
Maybe if you force your users to use a specific clock application (which provides to you a way to know it) you can do something. But you will end up with lose users.