By registering a listener to the beforesubmit event, you can validate a dialog.
... xtype="dialog"> ...
Serverside Validation
After defining a custom validating servlet, you can put the following code snippet in the beforesubmit area. It is important to call the servlet using async=false, to ensure everything is handled by the same thread.
CQ.Ext.Ajax.request({ method: 'POST', url: '/content.myvalidationservlet.json', async: false, success: function(response){
// do something
}, params: { ... } });