Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for determining whether content rendered on a display is actually visible to a user. In one aspect, a method includes generating and transmitting content presentation data that causes a user device to present, at a display and over a first time period, a content item that includes one or more content portions that are each designated for presentation in a respective first color. For a second time period, the respective first color of a particular content portion is changed to a second color. Visual representation data that includes a visual representation of the display during the second time period is generated and provided. Using the visual representation, an amount of the content item that was visible at the display is determined based on an amount of the particular content portion presented in the second color.