if radio button is checked once code example

Example 1: check if radio button is checked

$('#element').click(function() {
   if($('#radio_button').is(':checked')) { alert("it's checked"); }
});

Example 2: check if one of the radio button is checked

if ($('input[name='+ radioName +']:checked').length) {
           // at least one of the radio buttons was checked
           return true; // allow whatever action would normally happen to continue
      }
      else {
           // no radio button was checked
           return false; // stop whatever action would normally happen
      }