Glorious news, everyone! Units and cities now have the ability to detect stealth. Detection radius is based on visibility range and is different for land/naval/air units. If a hidden unit is within enemy's detection radius at the beginning of a turn, it will become visible to the enemy and his allies. Next turn, if it's outside the radius, the unit becomes invisible again.
Toggle 'show visibility range' (under the minimap) to see everyone's detecting range.
Also, if a team game reaches the maximum number of turns, the whole team with the highest combined SP wins.