Here are 2 SPAM images and 1 Normal image from my test.
I have also noticed that if it is a normal email sent from the site the hiddenField: appears above the CSD submit: Contact Us line and if it is a SPAM it is below.
I have a tracking code on the site and the SPAM "visitors" don't appear in the tracking at all, so definitely NOT from the site and definitely robo emails.
I hope this helps to troubleshoot that issue.
Thank you