Correct - all scarves were pre-printed. And the only real way to proof that Elvis gave you the scarf is to have a photo of him handing it to you. They sold the SAME scarves at the souvenir stands during the concerts.
I wouldn't doubt that there are a couple authentic signed scarves out there. I think I saw one sell years ago. This one is the pre-print kind. They were sold at concessions then and you can still find them today, sometimes being passed off for real.