Per Mother Jones:
"The center of the lid is pulled down by the vacuum, creating a slightly concave surface. If you're not sure that the surface is concave, push down in the center. If it doesn't push down, the container is sealed, but-if it does-the jar isn't sealed, and its contents have to be eaten or reprocessed promptly. If the lid is not concave, but pushes down and holds, the seal is questionable. In this case, remove the screw band and lift the jar by the edges of the lid. Care should be taken in doing this, because the jar may spill or break. When you've determined that a tight seal exists, remove the screw bands (if you haven't already done so). Bands left in place may become corroded, making the jars difficult to open.
Your containers of canned meat should be allowed to cool for 12 to 24 hours before being stored in a cool, dark, dry place. Remember that light hastens oxidation and destroys certain vitamins, and that dampness can cause the metal lids and closures to corrode or rust, endangering the seal.
Read more: www.motherearthnews.com/Real-Food/1983-09-01/Canning-Meat.aspx#ixzz1CD8GDHhx