1) Check for collaboration through formative assessment by asking each student (individually) what parts of the character lines they contributed and why.
2) Through formative assessment, question group members about roadblocks (or writer's block) they encountered while creating their open scene and address ways to work through those difficulties when applicable.
3) Allow students to read their scenes to the class. After scenes have been read, allow peer feedback to address how character choices could be shaped through different variations of line delivery. This peer feedback (with teacher guidance as needed) will serve as a formative assessment and help students understand if their scene, in fact, an open scene.
If there are a multitude of ways the lines could be delivered and therefore interpreted, then a successful open scene has been created.