Very difficult to tell. I got mine new and think there is a few websites detailing what to look for. Buttons having Burberry in them. The style if the letter B. the inner labels looking proper. Until you feel the item you will not know. Its the feel of quality and strong seams and buttons etc that will let you know if it is real.
Brand new should be £1200 in the Uk or $1600 I think is the USA price from official shops.
I got mine from a Celebrity and is the silver inner tartan print version.
Happy to sell you if you want but guess your in USA so would make it difficult for you to get a good feel of the item.
Good luck getting a real one for good price there is soo many fakes out there is very very difficult to be 1000% sure.
A search of the net should give you a good guide on these coats as I found one.