How to compare before and after state in whatever_submit($form, &$form_state)
First of all: You should migrate to Drupal 7 or 8. Drupal 6 is no longer supported as of Feb 24th, 2016.
If migration is not possible, perform security updates. The latest version of Drupal 6 is 6.38 released on 24 February 2016. Have you also heard about the Drupal 6 LTS initiative? I highly recommend reading the details https://www.drupal.org/project/d6lts, https://github.com/d6lts/drupal.

Answering your question, the simplest thing would be to pass the object with the data in the form:

$form['my_object'] = array(
  '#value' =&gt; $my_object,
  '#type' =&gt; 'value',
);

$form['some_field'] = array(
  '#default_value' =&gt; $my_object-&gt;some_field,
  '#type' =&gt; 'textfield',
);


Then in the submit handler you can compare values:

if ($form_state['values']['my_object']-&gt;some_field != $form_state['values']['some_field']) {
  // Value of some_field has changed!
}