Checking state of radio button

radio_button 'subject', 'valid_number', 'T'

<%= observe_field :subject_valid_number_t, :frequency => 0.5,
                   :url => {:action => 'add_element'},
                    :on => 'click' %>

add_element.rjs:

if I put page.alert('wtf')

everythign works fiine.

If I put:

page.alert(page['subject_valid_number_t'].value)

I get nothing. nada, no javascript errors, no rails errors, nothing.

How do I check the state of a radio_button? I don't care if I access
.checked or .value.