Opened cans are often mistaken for unopened cans, especially if the lid offers some resistance during an attempt to remove it. Here are some tips on how to tell the difference:
The metal strip that is wound off the during the opening process will be missing from an opened can. The can’s painted finish will differ where the strip was removed. Notice how there is no ‘blue’ at the top front of the example can that has been opened. The white paint is missing from the top of the back side also. The factory sealed unopened can’s lithographed finish is consistent from top to bottom. The key wind strip and tab are still intact.