Alternatives are available to chose the ReturnString:
Take the start page of your application and set the TestCmd to get /start_page. Set the ReturnString to a string contained in the startpage. With this strategy, you are monitoring that Apache Tomcat is serving your application.
Take the Apache Tomcat default startpage and set the TestCmd to get /default-startpage. In this case, set the ReturnString to a string contained in the startpage. This string depends on the deployed Apache Tomcat version, for 3.x it is Tomcat home and for 4.x and 5.x it is CATALINA. With this strategy, you are monitoring that Apache Tomcat is serving its default application.
Deploy a test application (which is not provided with the Sun Cluster Apache Tomcat agent) to Apache Tomcat. Set the TestCmd to get /startpage-of-the-application. In this case, set the ReturnString to a string contained in the startpage. With this strategy, you are monitoring that Apache Tomcat is serving your test application.
If none of above is appropriate, set the TestCmd to get /a-page-which-does-not-exists. In this case, set the ReturnString to a string contained in the Error Page. With this strategy, you are monitoring that Apache Tomcat is operational, because it registers that it must deliver a page that does not exist.
You can evaluate the different pages by connecting using a browser with hostname:port and specifying the different pages.