I think the #1 thing to consider is the price. I've seen bags on eBay for $200 dollars. Why would someone sell a $1500 handbag at that amount? If it's too good to be true, don't buy it!.... :)
Some more tips (and these work for any counterfeit handbags) Does the stitching match up? The logos should be perfectly centered and not cut off.
You can also do research and see if that purse you're interested in buying is even a real LV style. Counterfeiters frequently make up their own styles and plaster on logos in an attempt to excite the customer that they're getting a great deal on a 'rare' or 'brand new style' piece.