What is correct selector for radios to determine if ANY radio is selected?
Had same problem today. A slight modification of the negated value expression proposed on this comment: "Empty Radio Selection" on drupal_process_states() API page, did the trick for me!

'#states' =&gt; array(
   'visible' =&gt; array(
     ':input[name="radios-name"]' =&gt; array('!value' =&gt; false),
   ),
 ),


I just wanted the select list with the above #states entry to be invisible while no selection has still been made on my list of five radios (sharing name "radios-name"). 

Radios were also '#required' => true, in my case and all 5 individual radio-buttons have non-numeric string values. No #default_value is specified in my case either.

Disclaimer: I have only tested this on Chrome!