1.) there are sites where you can look up the model number on a watch, problem with this is, many of these sites are not owned or authorized by Casio. So some of these could be sites meant to mislead. But what i have found is that the more info it list, the more likely the site is giving as accurate info as it can. Casio does pull up most model numbers on their website once you click on watches and type the model number in the search box. Not sure if every single model number works that way, but i tested a few and they pulled up or similar numbers pulled up.
2.) If you can't hope on the net and need to decide right then if you are gonna buy or not, you can inspect it and look for a few things. Now keep in mind as you read this, those who make fakes could always upgrade what they do and make better knock offs, look what they have done with iphones and evos.... anyway, Most True G Shock watches have the Brand name Casio both on the front and back of them somewhere. Also aside from the back listing the things the watch can "resist" it should say "shock resist" on it also somewhere on the back.
3.)Every real Casio G-Shock should come with a box that has the full model number printed on the back (by the bar code). Look for the official Casio G-Shock logo too. And a partial model number will always be engraved on the case back. It's probably a fake if the watch doesn't have one, or if it doesn't match the photos in the watch archive.
4.) Safe spots to buy; Target, Amazon, Casio themselves.... ebay is hit or miss, Check background on the saler.
There's a ton of info out there ad i don't want to bog anyone down but do a search for fakes and more info on them and check this link http://mygshock.com/2008/01/15/how-to-spot-a-fake-g-shock/ , they have posted a lot of fakes that look real. Again, real ones start around 40 bucks and i have found them to max out around 6 or 7 thousands. These being diamond and jewel embedded.
But be aware, even the Bape G Shocks are being made faked. These two below....fake or real?
peace