United States Department Of Agriculture, Extension Service
After cooling jars for 12 to 24 hours, remove the screw bands and test seals with one of the following options:
Option 1. Press the middle of the lid with a finger or thumb. If the lid springs up when you release your finger the lid is unsealed.
Option 2. Tap the lid with the bottom of a teaspoon. If it makes a dull sound, the lid is not sealed. If food is in contact with the underside of the lid, it will also cause a dull sound. If the jar is sealed correctly, it will make a ringing, high-pitched sound.
Option 3. Hold the jar at eye level and look across the lid. The lid should be concave (curved down slightly in the center). If center of the lid is either flat or bulging, it may not be sealed.
Extracted from the "Complete Guide to Home Canning." First published: January, 1993. Revised: 2015.
Courtesy of the USDA and National Institute of Food and Agriculture