Typically used with: Insight.
In short: figure it out. Discern is all about sizing up a person or situation. A powerful general stands high over a battlefield deducing his opponent's objectives. A judge hears a defendant's case, and then determines if he's telling the truth. A powerful wizard sees through the disguise of an assassin posing as one of his servants. All of these are perfect examples of the use of the Discern skill.
Since Discern can be used for multiple purposes, the DL differs depending on the task. Mostly, Discern is an opposed roll.
Estimating is useful for determining the number of troops on a battlefield, revealing the market price of a gem, guessing a woman's weight, or figuring out how long building a ship will take with six laborers. The GM should decide the DL for each on an individual basis using the example difficulties chart at the beginning of the chapter. A character can include ranks from an Occupation which is relevant to the estimation (e.g. a farmer can estimate when crops are ready for harvest, a sailor can estimate the travel speed of a sailboat).
Learning combat plans
With a successful Discern check, you can determine your enemy's (or ally's) battle strategy before it unfolds. Some of the possible information one could Discern from battles include: a place, object, or person a combatant is guarding; the next plan of attack; a likely escape or fall-back point. Oppose your Discern against the combatant's Bluff (if you're trying to determine the plans of an entire group, use the commanding officer's Bluff).
With a successful Discern check, you can figure out specific things about people you meet. A DL of 10 will get you comparative skill (they're better/worse/as good as you at something). A DL of 15 will get you style specifics (the clothes they wear indicate they practice fire magic, etc.). A DL of 20 will get you limited statistic information (a ballpark of how many Health Points remaining, what percentage of their Magic Points are drained, or what's the neighborhood of their Endurance score, etc.) A DL of 25 or over gets you specifics — one stat (e.g. attribute, skill) per roll.
Discern attempts cannot be retried without spending a fate point, however, the character can try to Discern something different about the same subject.