To check if the canning jars have been sealed properly, press down on the center of the lid with your finger. If the lid is firmly concave and does not pop up and down when pressed, then the jar has sealed properly. Another way to check is to gently tap the lid with a spoon. If the lid produces a high-pitched ping sound, then the jar has been sealed correctly. Additionally, you can visually inspect the jar for signs of a vacuum seal, such as a depressed lid and no visible air bubbles in the contents of the jar.