Advertising the 3D effect is hard! Maybe if someone is watching a 3D broadcast, with a 3D TV and wearing 3D glasses would work, but even that situation doesn't convey how 3D looks without glasses.
You'll just have to find a way to try before you buy and Gamestop is allowing you to do that... the day before the 3DS launches in America.
Head over to Gamestop's 3DS launch page to see if a retail store near you will be holding one of the special 3DS preview events on March 26th. I'm guessing the suits are hoping to push a lot of preorders for the next day. It's not known what games you'll be able to play or what kind of access you'll get to the 3DS system but you can bet on seeing a ton of Nintendo's 3D handheld leading up to its North American launch on March 27th.
For continuing coverage on the lead up and launch of the 3DS, check out Game Revolution's new-fangled tag page for the 3DS.