常问问题: The game: advanced

Stealth units can be detected if they come within enemy's detection range. Detection is only performed in the beginning of a turn, and discovered stealthy units are announced in the Events menu.

Click on the 'show visibility range' button under the minimap to see your detection range (which is different for land/naval/air units. Detection range is directly proportional to unit's visibility range and also depends on that unit's type.