When Mormon introduced the plates of Ether in Mosiah, Chapter 8, Limhi asks Ammon if he can translate them. Ammon responds that he knows of a man who can, a man who is a seer. At the time, Mosiah’s name is not given, but it is clear in these verses that it is indeed Mosiah, and that Mosiah is indeed a seer.
The criteria of what makes one a seer is the possession of two stones fastened into the two rims of a bow. That description fits the interpreters that Joseph Smith received along with Mormon’s plates. Not simply the possession of the stones, but the ability to use them, made one a seer. Thus, Mosiah was a seer, and Joseph Smith was a seer.
There is no description of how Mosiah used these two stones to translate. However, there must have been some similarity to the way Joseph Smith later used them. These two stones were typically called interpreters in the Book of Mormon, and Joseph Smith used that term until it was suggested that they might be similar to the Urim and Thummim of the Bible. That term became the common way they were discussed, but, while it correctly associates these two stones with divinatory implements of biblical times, it allows for the confusion that the Old World Urim and Thummim somehow appeared in the New World. That was not the case, and these stones were different.
We will also hear of interpreters that were sealed up with the plates of Ether. These might be them, but even before the plates of Ether arrived, Mosiah had interpreters and knew how to use them. It was for that reason that he was already called a seer, and had already translated an old stone, before the plates of Ether and the Jaredite interpreters ever arrived in Zarahemla.