how can I spot a fake! HELP

The fakers have got so good, sometimes it can be very, very difficult to spot a fake. There are several things to look at: the shape of the bales, the shape of the buckles, the "slouchiness" of the bag. They shouldn't be stiff like cardboard, or really stand up by themselves, unless they are stuffed with something. The tag in and of itself, is very revealing.
This site : http://ateliernaff.blogspot.com/2006/03/balenciaga-hardware-devils-in-details.html is awesome in showing what to look for. There are also many kind people who will help you authenticate Balenciaga in the "Authenticate This Balenciaga" thread.

White is also a colour that is released every season, so if you purchase from a reputable dealer, you can be assured you are getting an authentic handbag.

Good luck on your search.
 
I agree with Bridget. Ateliernaff is run by a PF'er and is a great source. If you are looking at a bag on Ebay, post the link in the "authenticate this" area - some of the girls here are really knowledgeable.
 
The bag on the link you posted is fake. You can tell by the the leather. It's supposed to be smooth and distressed, the one you linked to looks very plastic to me. Also the stud-thingies (can't quite remember what their called) in the corners is to close to the buckle(?).

The price is also an indication that it's fake, even though white is a color that's not so hard to get. It says new with tags, and there's no rips or anything as I can see, so if I where you I'd look for bags for at least twice as much!

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