Vladimir is correct. "cphaprob stat" will tell you via the CLI.
From within the GUI there are a couple of ways.
1 - In R77.x or earlier, this is shown within Smartview Monitor.
2 - In R80.x and later, go to Gateways and Servers, highlight the GW/Cluster in question and click on Device & License Information at the bottom. This will bring up a Smartview Monitor view and ClusterXL should be part of the listing. (Unfortunately I do not have a cluster in my lab, so I cannot 100% validate this.)
3 - In R80.x is on Mgmt, you could run a script against the GW using the above cphaprob command. Again, on Gateways & Servers, Scripts. The body of the script is the command to run. The Task will complete and show in the bottom left corner. Click on Details of the task to see the results.