Ways to find out if php form is being used for spam?

If you don't have access to the server logs, what is a reliable way of checking to see if your php form is being used to send spam?

I know if the form is being used to send spam to me, but I want to make sure it is not being manipulated in some way to send spam to others.

Edit the script to have it log all the requests it processes in a separate database that you do have access to.