Instead of asking questions of the seller to prove it's real, put yourself in the position of someone trying to sell a fake one. When you're asked a couple of questions you're not going to hold your hands up and admit it are you, you're going to lie about it. At a guess:
"It was an auction at work that we did for (insert random charity)."
Who do you work for?
"Cash Converters, at their head office, the shirt is one of the samples we were sent to judge the appearanc of and this is the one we chose."
At least that's the way I'd go if I wanted to explain having got it in advance of the official release and wanted a convincing story.