where is the submit handler for the Drupal Commerce Review page?
All checkout page forms are built using the checkout pane info David Thomas indicated through the function commerce_checkout_form() in commerce_checkout.pages.inc. The validate handler does double duty for the checkout form, validating and actually submitting data on a pane by pane basis to capture valid data as soon as possible. This function is aptly named commerce_checkout_form_validate(). Your best bet will be to log debug messages using watchdog instead of displaying them on the screen via drupal_set_message(), because we do some status message processing in there to put messages inline with the panes that generated them - that can make it hard to force a debug message to show when you expect it.