Who will tell me how to identify the real louis vuitton bag?

who will you tell me how to identify the real louis vuitton bag? I have just received my item Louis Vuitton M40409 in Gray from this site http://www.louisvuitton-gucci.com it comes with yellow dust bag,serial and model numbers,gift box,care booklet,cards and Tags. the price is quite cheap,only $249,I am wondering is it a real one? HELP ME!