There is not a command for it, however there should be a an X-header which is populated with the IP of the GUI node.
So if you connect to the non-GUI node, the response should contain this X-header directing you to the right GUI node.
Thank you for the answer. I don't have this kind of information in the header. Here is the kind of header I get :
HTTP/1.1 200 OK
Date: Thu, 06 Aug 2015 13:53:22 GMT
Sometimes, I have the following reply when using the REST API in some hosts: another gui is in use:https://XXXXXX
Does it mean that it's a slave?
As mentioned, you will only get the header when you attempt to access the *non-GUI* node.
If you got a 200 response, this leads me to believe you were accessing the GUI node.