Here are some ebay guides for some of the more commonly faked designers.
Louis Vuitton - Fashionphile has a wonderful ebay guide that is several pages long:
eBay Guides - How to SPOT fake LV LOUIS VUITTON authentic Guide 1
Chanel - Burberry_princess has a great guide:
eBay View About Me for burberry_princess
Coach - to.much.stuf has a guide... but no pics. I'll try to find a better one:
eBay Guides - How to spot a fake COACH bag
Gucci - Here's an ebay guide from aegiale:
aegiale eBay Store About My Store
Balenciaga - luv2shop-hate2spend does an excellent job explaining things:
eBay View About Me for luv2shop-hate2spend
Prada - A guide by peskipiksipesternomi:
a peski shopping habit eBay Store About My Store
These are all I can think of right now... but at least it will get you started. The best you can do is just keep reading up on it and to go to a store and examine a designer handbag in person so you understand what they look like.
If anyone else knows of any good guides... please share!