I'm by no means a handbag expert, but I've sold quite a few. The proof is in the details. You have to look at everything from the material they are made from, the stitching, the lining, the straps, even down to the badge itself and how it is laid out.
For some reason, women are born with this innate gift of knowing when a purse is a knock-off. When a purse comes in sketchy, I ask a few customers what their take is on it and we usually come to a pretty accurate conclusion.
Some of those designer imposters actually can bring in some decent money also, some people can't afford the real thing and prefer the fake.