I Just Bought A Tiffany & Co. How Can I Tell If It Is Fake?

I just bought a Tiffany & CO off craigslist and went to the peoples house and it seemed legit… but now that i got home it has that silver almost white appearance. how can i tell if its real or not is there some kinda of at home chemical i can dip it in? to see if its real or not i dont want to be giving my GF fake jewlery