A sharing client device shares content with viewing client devices. A sharing module operating on a sharing client device and on the viewing client devices facilitates the creation and analysis of viewer snapshots, permitting automatic determination of whether viewers are up-to-date with their display of shared content. Based on the analysis and on a policy of the sharing module, the sharing module can take a number of actions, such as generating and displaying a viewer summary that a user of the sharing client device may view to determine whether, or to what extent, the viewing client devices are up-to-date in their displays of the shared content.