1) open the page with the form that sends the email
2) go to Insert->Form->Hidden to add a hidden form element
3) give it a name and a value (it doesn't matter what, so call it "hiddenName" and give it a value of "hiddenValue")
4) edit your email body and add this to it somewhere: <?php echo($_POST["hidddenName"]); ?>
5) make sure your hiddenName is the actual name of the hidden form element you added.
Now the next time you get a spam email see if the value from the hidden name appears where it was referenced. That would confirm that it is coming from that form.