How do one know which folder is created by which Android apps?
If you open a file as Context.MODE_PRIVATE, then the file is created in /data/data/app_packagename and is only accessible to your app. You can see the permission and owner of these files in command line.

adb shell ls -al

drwxr-x--x    1 app_28   app_28        2048 Dec  6  2011 com.android.mms


But files in sdcard is public to all user, so you cannot find out which folder belong to which app.


  Files saved to the external storage are world-readable and can be
  modified by the user when they enable USB mass storage to transfer
  files on a computer.


See this link for details.