When using the UISelectOne then you cannot today know which index the user selected, whether it was the first, the last, or some other in between.
You can only get the value, and then from the value you need to determine yourself if it was the first, last etc.
Also due to bug
FORGE-552 the returned value can be invalid, and you would not have a way of know exactly what was selected.
The same thing would be nice on the change listener, where its applicable.