Radios button in form arrays How can i check which radio button has been checked from a list of radio buttons ? Is there any key in the form array which holds the checked radio ?

I am using an ajax call for a bunch of radios buttons and i would like to do some "checking"  only if the right radio button was selected. 

As an alternative , how can i append the "#ajax" parameter to only one radio button in a list of radio buttons ?
You can check the element's value in $form_state['values']. Add the full parameter list ($form, &amp;$form_state) to the form builder function.

For example:
function mymodule_something_form($form, &amp;$form_state) {