How can you spot a fake marc jacobs stam purse?

I'm looking at a marc jacobs stam purse from www.topdesignerhandbags.net. the company claims that they are all authentic, however the bags are used and they refurbish them. the one shown has red suede lining and it has a white string on the tag hanging off. However, i don't know if that is the original tag since the bag is used. help me please!! thanks!