Tips or Tricks: Identifying if there are Opened Documents
Just by looking the taskbar I think you can't. But you can set an easy shortcut and if you click on the application you want to view and the press the shortcut you can see all the windows opened (active and minimised) for that application. 

In System Preference > Mission Control, you can set a shortcut for Application windows. Just try it once and with time it will become smooth for you.