If I received previously used kosher ceramic serving dishes as a gift, but I am unaware if they were used as meat or dairy, how can I kosher them?
You can decide today whether you want them for meat or dairy going forward. That’s it.
Even if used for hot food? Is this because eno ben yomo? What about if not sure if guests used for treif?
Primarily it’s because of the glaze on the dish that never allowed anything to be absorbed into the vessel.
Nevertheless, Once you determine what you will use it for (Dairy or Meat) you can not interchange it again.
Thanks. Is that also true if you’re not sure if a guest used for treif?
Yes. The same