If you have "Hey Siri" enabled in Settings>Siri, then your iPad is listening for that specific phrase and only that phrase. Once spoken ("Hey Siri") and thus activated, it will then listen to whatever you say after that until it is disengaged (either by pressing the home button or by completing the query). In summary, you are aware if it's listening or not by the UI on the screen. You can also easily disable "Hey Siri" if it makes you feel better.