I believe seal engraving was done by the use of a small wheel driven by a bow in the manner glass engraving is still done today.
The principle of a lathe is as ancient a bit of tech as there is, powered by a bow or a treadle. Oil or water with powdered abreasive are splashed on the wheel and work and a steady hand and good eye are the rest.
It would be hard to tell without super close ups old from new.
Wear and "feel", combined with experience, are the only way to tell, and then you could get into a case of dueling experts!
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