Form Answer to Trigger an Alert

We created a form that allows users to download a piece of our content. We included a question in that form that says 'want a demo?'.

If the lead checks off 'yes',  I obviously want to send through the demo request to sales. I don't want to send someone through that simply filled out the form. I don't see a constraint that says 'filled out form', 'checked this box' or 'answered this question'.

Does anyone know what to do here?