There are different ways to see other hosts rather than just ones on which you have installed a OneAgent. For example, with VMWare integration you can easily get a list of all VMs that don't have OneAgents yet via a filter named "Monitoring candidates" on the host list. You can also see services making calls to Unmonitored hosts and get information about the calls being made to track down hosts that are part of an application's service "flow".
But generally speaking, the usual practice is to get an inventory of the hosts known to be used by a application (or applications) and roll out OneAgents methodically from the Web Server to the backend and use the other techniques to find gaps in your monitoring.
HTH,
dave
