Judge of intent is a social skill used by dwarves while chattering amongst themselves and avoiding work. This is an important broker skill, as it allows the broker to tell how a trader feels about a particular deal.
To train this skill, let dwarves idle and chatter amongst themselves.
If your broker is Novice or better in this skill, you will get messages about your trading partner's willingness to trade. See Trade#Merchant_mood. It is possible for a dwarf who starts with Novice skill to become rusty by the time the dwarven caravan arrives, but a single small trade will remove the rust and allow the trader's mood to be gauged.