XEP-0092: Software Version allows you to discover the software version and operating system associated with another XMPP entity.
SoftwareVersionManager softwareVersionManager = xmppClient.getManager(SoftwareVersionManager.class);
SoftwareVersion softwareVersion = softwareVersionManager.getSoftwareVersion(Jid.of("example.net")).getResult();
If you query a server, you will obviously get the server software. If you query a full JID, you will get the client software associated with the connected resource.
You can set your own version, if you want other entities to discover your version:
softwareVersionManager.setSoftwareVersion(new SoftwareVersion("MyClient", "0.9"));