Often it was very difficult to authenticate them as genuine as the following example shows.

In Livy's 'The War With Hannibal' Hannibal manages to kill one of the Roman consuls and capture his signature ring. He then sent a copy of documents stamped with the ring to a nearby town with orders purporting to be from the (now dead) consul to admit him and his troops that night. Obviously Hannibal hoped to take the town by surprise. But he was thwarted because the populace had luckily received prompt notice of the death of the consul.

The implication is that the townsfolk had no means of checking the authenticity of the ring and, had they not received word of the consul's death, would have fallen for the ruse. So if there were reference copies they were not widely available.

If there was a question of authenticity, of forgery, the original seal could be consulted or an accepted document that also had been sealed. There would likely be more than one copy: just like today, where all parties to an agreement get a copy. There are , for example, several copies of the Magna Carta.

Even in the most basic of transactions, paying taxes, there were copies: the old English system was to use tallys, which were short sticks of hazel wood. The amount was entered by a series of cuts into the wood, the names were written on each side, then the tally was split. If there was a question of the amount being paid, the two halves could be put back together- any later alteration would be obvious, since it would be on only one half. It's a famous loss to economic history, that in 1834, after the system had been dropped, the giant accumulation of tallys was burned, heating Parliament. It set the building on fire, and others too, which was bad- but the amount of data that was lost was probably even more important.

Might be worth adding, that forgery was considered a serious crime. Look through the Newgate Calendar, and you'll see they were hanging forgers into the late 18th century. Personal notes in lieu of cash were common, and often passed around like cash, so forging them was a great temptation.