Do they “pop” like metal lids, and how do I know they are sealed?

Sorry, they don’t pop… But you will easily know they have sealed once you remove the metal screw band after the jars have cooled. After doing so, simply grasp the rim of the plastic lid with your fingertips and lift. A failed seal will lift easily from the jar without resistance. The same for a jar that loses its seal in storage. A quick check of jars in storage, by gently lifting or running your hands over sealed jars will identify if a seal has failed. It is our experience that once sealed, few jars lose that seal, even after several years.