One technique is radiocarbon dating:
https://en.wikipedia.org/wiki/Radiocarbon_dating
EDIT: For artefacts made entirely out of stone, you can date objects in the sediment around them to get an approximation.
https://en.wikipedia.org/wiki/Radiocarbon_dating
EDIT: For artefacts made entirely out of stone, you can date objects in the sediment around them to get an approximation.
If the stone in TFA gets buried when _our_ civilization collapses, the stone would now probably date to 2020 AD instead of whatever it should have been.