If you're shopping online for your prom dress and suddenly stumble upon a site that has prices that are deeply discounted... think twice before you click “buy.” The dress is a probably a fake.

Overseas websites (many of them China-based) are targeting prom girls by stealing images from top prom designers like La Femme, Jovani and Tony Bowls, then claiming to have those exact dresses for sale. In reality, it's a classic bait-and-switch.

The dress that gets shipped to you is a cheap knock-off with fit problems that looks nothing like the real thing. Or, it never gets shipped at all.

That's what happened to Dallas-area teenager Joi Mainoo, who forked over $1000 to an overseas website for a prom gown and after-prom dress. Her dresses never arrived... and she ended up spending over $500 on a new prom dress at a boutique just four days before the big night. Ouch.

Federal agents are investigating, but it's proven hard to shut down the websites. New ones pop up weekly, and faster than you can say “beaded bodice.”

"Unfortunately, if the price is too good to be true, it's a fake prom dress," said Jon Liney, president of DressGoddess.com, in a release. "Prom dress designers determine the price of their dresses and every authorized prom retailer, both online and offline, must sell dresses at the Manufacturer's Suggested Retail Price or MSRP."

Designer Sherri Hill told Dallas News that she began putting serial numbers inside her dresses to guarantee their authenticity.

Prom e-retailer giant DressGoddess.com has devoted an entire section of their website to educating consumers about fake dresses, and guarantee their dresses are the real deal. They're even offering a 20-percent discount on accessories to consumers who take their prom pledge to buy in the U.S.

Next, tried-and-true tips for telling if the website you're browsing is a big ol' fake.