Just this week, Hyperixa released a neat little app called IPSW, which could be used to see if a particular iPhone or iPad firmware version was being signed by Apple at the time of checking. The IPSW app was first made available for jailbroken iPhones and iPads via package managers such as Cydia, Sileo, and Zebra.
Hyperixa’s firmware signing status checking app IPSW also available for non-jailbroken devices