if one radio in the list is checked or not

if one radio in the list is checked or not

Hello !

How can i know if  one radio button is checked or not  in a radiobuttonlist   name="payment_method"   ?

  1. <input id="payment_method_1" type="radio" name="payment_method" value="payment_method_1"  />
  2. ........
  3. <input id="payment_method_5" type="radio" name="payment_method" value="payment_method_5" />

thank you