Minecraft wiki entry for Setting_up_a_server, associated text: Open Windows Control Panel, find Java (it may be inside the Programs category), and click on Update Now.
Visit http://java.com/en/download/installed.jsp. This will perform an automatic version check from your browser. However, the Google Chrome and Firefox browsers do not run Java content and therefore cannot check Java through the browser.
Open a command window and enter the command java -version. If a version number is reported, then check the Java website to see what the most recent version number is.