You can do that by adding a change listener. Use this line of code to add the listener to your select:
Code:
MyClass.prototype.createSelect =
function() {
var select = new DwtSelect(....);
select.addChangeListener(new AjxListener(this, this._changeListener));
}; And write a method to handle the change event.
Code:
MyClass.prototype._changeListener =
function(event) {
// Do something interesting here.
alert('Changed');
};