How do you know when a 510 cartridge is empty?

How do you know when a 510 cartridge is empty?

You'll know your vape is empty when you can see there is no more oil in it, you struggle to produce vapor, and the taste of the vapor deteriorates.

KVAP is the Manufactuer In China and Produced Empty 510 Cartridges For Extractors Or Wholesalers or Retailers.