Burn in is a big danger on an AMOLED demo unit. If they left the same picture on for long periods of time, it almost has to have it.
As someone else said, pull up a white page and look for ghost images. I have seen some store demo units on eBay that have burn. I used to see a bunch of Samsung Galaxy's with it.
You might to check to make sure it's not nicked up either. I imagine it's been dropped a few times.