In a real life situation It is most of the times quite clear but there are some symptoms (in some cases) that may lead you to think that there is a respiratory activity (eg Gasping); if you apply the golden rules of BLS protocol you'll take the right decision with very tiny chance of making the wrong evaluation. The chest motion during the breathing process is a good sign for the evaluation (in the case of gasping there is air coming out of the month but if you look at the chest you'll see no movement or abnormal and "aritmic" movement) as Radgirl well explained!