You can’t 100% can’t.
It is asked in this Google I/O 2012 video and the Project lead for the new notifications declares that you can’t.
Edit
2016 update: Now you can check it, as said in this Google I/O 2016 video.
Use NotificationManagerCompat.areNotificationsEnabled()
, from support library, to check if notifications are blocked on API 19+. The versions below API 19 will return true (notifications are enabled).