Very simple, start a JBoss instance on each system which are in the same sub-network.
You will see a log message like 'Number of members: 2' in each log (start the second if the first is up just to see it easier)
If you want to protect against accidentally clustering you should start the instances with the option
'-g <a name> -u <a multicast ip address> -m <a multicast port>'
in this case the cluster-partition is different to the default ;)