Check code and a uniform resource locators (URL) are sent from a server to a computer. The check code, when executed on the computer, lists a reference to the URL so that an attribute of the listed reference to the URL varies depending on whether the URL is in a list indicating URLs visited by the computer, and forwards to the server, based on the attribute of the listed reference to the URL, an indication as to whether the URL is in the list indicating URLs visited by the computer. Content to be sent to the computer is selected based on the indication.