A technique is provided to determine whether a region within a web page is viewable to a user through a browser window. Often, browsers will only show part of a web page at given time, creating a difficulty in establishing whether a region of the web page, which may be an advertisement for example, is in view. This is addressed by providing one or more test features within the region, rendering the web page, monitoring a behavioral characteristic of the test features and determining whether the region is in view based on the monitored behavioral characteristic, wherein the behavioral characteristic varies according to whether the test feature is currently being displayed. One example of a behavioral characteristic is a frame progression rate. Browsers will typically redraw elements of a web page at a higher rate if they are currently in view through the browser window, and this characteristic can therefore be used to determine whether the test feature, and thus the region, is in view. The present invention finds particular utility where the region contains an advertisement, as it allows an advertiser to discover whether the advertisement has been seen by users.