Look at the left side of the picture box on an early Base Set card. A small black circle with "1st Edition" means the card is from the very first print run.

Unlimited cards do not have it. Same set, same art, same attacks, so the stamp is how collectors tell them apart.

Two things to check. The stamp should be crisp and centred. And look at the whole card: a clean stamp on a heavily worn card is still a worn card. That is why we show every card up close and grade it NM to DA.