If its been irradiated with neutrons/protons/heavy ions... I could do an activation assay for induced radioactivity.
(I will however change you ungodly amounts of money to do so)
If its been hit with gamma/x-ray. There is really sweet fuck all you can do to tell. Mainly because gamma/x-ray irradiation does not really change anything of large importance (It creates ions pairs as it slows down in the material, causing weird breaks in big long molecules like DNA and RNA that prevent the bacteria from reproducing, however, if I took a 'random' string of RNA of some N base pairs and another random one of the same N basepairs, one irradiated and one not, there'd be no way to tell the the difference)
EM radiation is an event...light is EM radiation just the gamma the use on food. When you turn off the light bulb, the light no longer remains does it? Particle radiation is different, but is not used in food and is best left to synchrotrons and eggheads to play with.
Medical devices are often gamma sterilized, its an efficient and surefire way to guaranteeing something like a packaged needle is sterile.
