Rachael and Gretta break down how to spot a good designer knock-off and a not-so-good one and tell you when it's worth it to splurge on the high-priced real thing. Dior-like sunglasses are available on the street for $10. The real Christian Dior glasses are $340. If you pick the real ones up, they're solid, the logos are also where they should be, and, most importantly, they protect your eyes. The cheap, fake ones sold on the streets of New York might be fun and make you look like a movie star (and if you're prone to losing or breaking things, they're a bargain!) but you can definitely feel and see the difference.

For handbags, it's all about the fabric and finishing if you're trying to spot an illegal knock-off. Look closely at the metal details and the logos; those things give it away when they're either printed wrong or the stitching looks off. You can also tell the difference between leather and synthetic right away.

Some knock-offs are really high-end so you have to take a closer look, particularly on the inside where they usually don't have the exact details as the real one. If you want to buy something online and take your chances with an unauthorized retailer, check out the real thing at a store beforehand. Hold it, feel it, open it up so you'll have a point of reference when you get yours.

When it comes to shoes and clothing, many big-name designers have created "fast-fashion" (aka more affordable but equally as trendy) lines at stores like Target, Wal-Mart and H&M. They're designed to look like the high-end, more expensive items but they're much more affordable.

Ultimately, the lower price means cheaper raw materials like synthetic fabrics and plastic. "An important thing to remember," reiterates Gretta, "is the original, more expensive items are better made."

So does that mean you should or should not spend the money? Here's a rule of thumb: If you find a shoe or dress that's a classic like a wedge shoe or a wrap dress that you will wear for more than one season, it's worth it to invest in the high-quality item rather than the fast-fashion one because it will last longer. "I went for the pricier Diane Von Furstenburg dress," says Rachael. "I live in this dress and I need it to last!"

Fast-fashion items are great for trends that come and go. Do you really see yourself wearing that hot pink mesh top again? Aren't you glad you only spent $12 on it? If it's something you only see yourself wearing for a few months, save your pennies.