There are a number of ways to tell when your carbonating cylinder is empty:
1. When you press the carbonating button you don't see any gas going into your bottle of water
2. Even after repeated presses your water is not carbonated
3. You hear a sputtering sound when you attempt carbonation
Pro-tip: SodaStream recommends keeping a back up carbonating cylinder handy so that when one cylinder runs out, you simply swap the empty cylinder for a full one. That way, you'll keep enjoying fresh, fizzy drinks without interruption and can exchange the empty carbonator via courier service or in a store near you.
Also take a look at this video that may be helpful.