Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for detecting errors that can occur in third party content presentation and verifying that third party content provided by a content provider to a content platform is actually displayed and is visible to the user when the content platform is accessed on the client device. Methods can include receiving, from an application executing on a client device, a request to generate a digitally signed token that is used to validate whether a particular content item displayed at the particular portion of the display is a third party content item. A digital watermark embedded at the particular portion of the display can be extracted and decoded to obtain data for attributes that are descriptive of the particular content item. A digitally signed token can be generated using this data, and the token can then be provided to application.