how can you tell if

There are a several things to take into consideration. There is an authenticate this thread you could check out.
I advise friends to buy from either the Coach boutique, Coach factory store or Coach online to ensure they are getting a real one. There are some good sellers on ebay but there are also some rip off sellers on ebay.
Good luck!
 
Bags sold in department stores are authentic. The only way a department store would get a fake is if somehow someone bought an authentic bag and returned a fake one in its place (as a scam to get something for nothing). I have heard of this happening but I'm not sure how common it is. For the most part you should be safe buying a bag at Dillards, Macys etc.:yes: