Identify Leads from Online form and Enter Workflow

I have a workflow setup for our leads/customers who enter catalog requests on our site. When someone fills out the form, the online form assigns them to our catalog request lead source and they enter the workflow. It's all worked perfectly.

One issue I've found, however, is customers who come in from our PPC Adwords links. We pass a lead source through the URL which is associated with the ad campaign. This lead source takes precedence over the one assigned by the form itself. So these customers are not getting put into my workflow on creation.

I'm wondering if anyone knows of another way of identifying leads who have filled out a certain form, other than by the lead source. Then I could use this alternate criteria for my workflow instead of the lead source.