According to NCHFP, there are several ways that you can make sure your jar is sealed properly:
At eye level, notice the lid. If there’s a depression or if it is concave, your jar is sealed properly. If the glass jar lid appears flat or bulging, you’ll have to reseal the jar.
Put your finger in the center of the Mason jar lid. If the lid springs up when you let go, the jar isn't sealed properly.
With a teaspoon, tap the lid. The sound of a ringing canning jar indicates an airtight container. If the jar isn’t sealed, it will result in a dull sound.
Note: Please check the seal only after 12-24 hours of processing.