Sadly, when your elf bar is finished, you will see the elf bar flash, indicating that the device is finished and will no longer work.
When you draw on the bar, no vapour will come out.
You will have to use a new device and dispose of the flashing elf bar in accordance with your local authorities' guidelines.
Make sure you have removed the sticker at the bottom! As this is commonly mistaken for a faulty elf bar.