How to know my nexus device is file encrypted? From android n new feature called file encryption is supported.
How to identify if my device is file encrypted.

If i create any new file inside /data parition using touch . Will i will see that file when i do ls -l.

Will file name will be shown as secured name when doing ls -l after creation of file, if the device is file encrypted.
If you have 'Developer options' enabled you may see a 'Convert to file encryption' option. If you see this option it implies that you are not using file encryption. Clicking this option shows a warning stating:


  Convert data partition to file-based encryption.
  
  !!Warning!!. This will delete all your data.
  
  This feature is alpha, and may not work correctly.
  
  Press 'Wipe and convert...' to continue.


I don't think that any devices that are upgraded to Android 7.0 will use file encryption automatically as it requires wiping your device in order to turn this on. Any new devices (such as the new Nexus devices coming later this year) should use file encryption by default.