I think Magicman38 is actually asking how to tell how old the shed is.
Not sure maybe I read the post wrong.
If that is what he is asking after it has been on the ground more then 2 years it is hard, but most sheds will be chewed up by small game animals before that time so most of the sheds found are 2 years or newer. Depending where they were dropped. Sheds from this past winter should still have fairly white bases on them while older ones will have started to be bleached white by the sun an the bases turning a darker colour.
Magicman38 if you have a picture post it and I'll try and give you a better idea as to the age. If this is what you are asking.