See the complete code below. The checkboxgroup change event is not consistent /w the field change event.

Code:
Ext.create('Ext.Window', {
    width: 300,
    height: 300,
    layout: 'fit',
    items: {
        xtype: 'checkboxgroup',

        columns: 2,

        listeners: {
            change: function (cb, oldVal, newVal) {
                alert('checkboxgroup: ' + cb.boxLabel + ': ' + oldVal + ', ' + newVal);
            }
        },

        items: [{
            boxLabel: 'label 1',
            id: 'check1',
            listeners: {
                change: function (cb, oldVal, newVal) {
                    alert(cb.boxLabel + ': ' + oldVal + ', ' + newVal);
                }
            }
        }, {
            boxLabel: 'label 2',
            id: 'check2',
            listeners: {
                change: function (cb, oldVal, newVal) {
                    alert(cb.boxLabel + ': ' + oldVal + ', ' + newVal);
                }
            }
        }]
    }
}).show();