The Ascension Project
When a combat starts, if a character is not aware of opponents they are surprised. The GM determines who is aware of whom at the start of a battle with Perception checks, or other checks to see how aware a group are of their opponents.
The Surprise Round: If combatants get the drop on their opponents, a surprise round occurs before regular rounds begin. In initiative order (highest to lowest), combatants who started the battle aware of their opponents each take one action during the surprise round. You can also take free actions during the surprise round.
Unaware Combatants: Combatants who are unaware at the start of battle don’t get to act in the surprise round. Unaware combatants are flat-footed until the start of regular rounds.