How can I know if my iPhone had been erased?
When the device is erased, you should receive an email notification informing you that the device has successfully been erased. This is regardless of whether the device was online or offline when you clicked Erase, since the next time the device is online the device will be erased and an email will be sent.


  
  If your device is online, the remote erase begins. A confirmation email is sent to your Apple ID email address.
  If your device is offline, the remote erase begins the next time it’s online.
  


Source: http://support.apple.com/kb/PH2701

You will be unable to track the device through Find My iPhone if you erase it.


  Note: If you erase your iPhone, you won't be able to use Find My iPhone to locate or protect it, but your Apple ID and password will be required to reactivate it.


Source: http://support.apple.com/kb/TI79