[Zope] How can I check a user_acl folder to see if a user already exists?

Here's the setup: I'm trying to add users to my site by having them fill out a form with the following fields: Username Password Confirm (password confirmation) I want the form to post to a python script that will check the folder and do a redirect to another document depending on what happens... I don't know Python that well. I've tried doing it in dtml with no results. Can someone help me with a little example code or at least point me to a product that does this? Thanks in advance. -Rob