There are two ways that you can preview. You can either click the "Preview Form" button when you're creating the form, or you can preview it just before you send it out.
Previewing Form from admin
You can see what a Lineup Form looks like as you create it. When you're on the Lineup Form's admin page (either Form or Sharing tab), click on the "Preview Form" button in the top-right of the page.
Previewing Form before sending
Just before you send out a Lineup Form (from the Actions menu on the People page, or from the context menu on an individual profile),there will be a button: "Preview Form."
You will then see a preview of your Lineup Form that everyone will receive.