Also do a little bit of research on the bag from the outset.
For instance, a real Louis Vuitton will never ever have the LV monogram on a seam. Plenty of the fakes do however. For instance, here's one ending soon on eBay:
It's up at $800, but it's a fake: note that plenty of the LV's are cut by the seams. If this occurred in the real LV factory, the bag would be destroyed upon inspection.
But here's a genuine one: http://cgi.ebay.com.au/Authentic-Louis-Vuitton-Monogram-Speedy-30-Like-New_W0QQitemZ280228759249QQihZ018QQcategoryZ63852QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
As Danica herself said, this person has kept everything including the original receipt.
So find out details of the real bags. With a Chloe Paddington bag for instance, the fakes often have serial numbers embossed on a leather tag inside the bag. Worse still, they're all the same serial number. Ask the seller for the serial number and type it into Google. Odds are that if it comes up, you're looking at a fake. If they refuse to tell you the serial, don't buy.
Don't take the inclusion of a carry bag as a sign that a bag is genuine, these can be faked too.