Medallions of thoughts were carved magical trinkets, generally taking the form of Human skulls inlaid with a permanent Divination enchantment within. An attuned wearer of the medallion could use its magic to cast a Spell which allowed them to detect a creature's thoughts, provided they knew the same Languages or had Telepathy.
This magic allowed them to pry beyond surface thoughts, but the targeted creature would know the caster was probing their mind, and could cast them out if they had sufficient mental fortitude. Medallions of thoughts typically had a limited number of charges, which recovered daily when not in use.