There are several things you can do to ensure your Prada's are authentic and not a fake.
1. Check that they are "Made in Italy" all Prada sunglasses are made in Italy by Luxottica who is the manufacturer for Prada. Prada's are not made anywhere else, so if you told they are, it's a lie and you have yourself a fake Prada.
2. Your Prada should come in a box and the following included, a Prada cleaning cloth, Prada Case, Luxottica info booklet and of course, the Authenticity Card, which comes in a envelope and is white and has a credit card shape and feel to it.
3. Also, on the inside of the lenses there will be a serial number engraved on the lenses, this serial number is unique to your Prada sunglasses, fakes do not have this.
4. Also one of the temples will have this same exact serial number engraved on to it, this is to confirm that the lenses and frame hasn't been altered in anyway and that they go together.
5. Check the inside of your temples, on one side, it'll have a printing that says something like this "SPR 54IS 5AV3M1 57  18-135" Below is what they stand for...
-The S in SPR stands for Sun, in RX frames, it'll be VPR, the V stands for Vision.
-The 54IS is the model number of your frame.
-The 5AV3M1 is the color code for the frame. Prada color codes contain 6 number and letters, the first 3 is the frame color code and the last 3 are the lens color code.
-The 57  18-135, it'll be 3 numbers, this is the measurement for the frame and is measured in Millimeters, the first number 57mm is the lens width, 18mm is the bridge width and the 135mm is the temple length.
6. If your Prada sunglasses claim to be polarized, don't just take their word for it. You can do a simple check if they are polarized. All you need is your computer monitor and your sunglasses! Put your sunglasses in front of the computer monitor and turn it to the right, while looking through the lens at the monitor. At a certain angle, if they are polarized, the screen behind will seem to go dark. If it does not do anything, then it is not polarized. If the sunglasses claim to be polarized, but don't show polarization in this test, you either have a fake on your hands or an error was made during manufacturing. I've seen some frames come out of the factory with a faulty polarization, so don't rule that out.
7. Check the fonts on your Prada sunglasses. Many times, fake Prada sunglasses get this part wrong. They attempt to do it as close as possible, but rarely ever get it 100% right. If you have an authentic pair available, compare the fonts used.
8. The best thing to do is to go to a authorized Prada dealer in your area and compare the one you have with the one in the store and they should be more or less identical. As mentioned, certain specifications printed on the interior temples could be on the opposite, but that's common across frames as they could have been an error with that particular batch.
You could ask the person in the store for their opinion, but definitely take it with a grain of salt. Stores are staffed with sales people, after all, and helping you to determine whether the frame you bought at a discount from their prices isn't making them any sales! Even mentioned the words "bought online" seem to automatically trigger them to tell you that your frame is fake-but, you're in luck, you can buy the one they have there! Sometimes you'll find a gem of a salesperson who can truly help, but be aware some are less than knowledgeable. So I would suggest going to more than one place and getting multiple opinions.
9. Buying your Prada's online? There are lots of fakes being sold online, but there are also legitimate sellers as well, so don't assume they are all fakes because it is being sold online.
You should look at things like if they are standing by their products, such as "100% Authentic Guaranteed". People who are selling fakes may not want to put that because it isn't authentic, but sometimes they might, look closely and lots of times maybe at the bottom or somewhere around the page, you might see something like "replica Prada", "looks like Prada", "similar to Prada", "good Prada copy" as if they mention that, then they can't really get in trouble because they told you it was a fake vaguely, so read their page carefully.
If you're buying Prada sunglasses on eBay or Amazon, you should look closely at their feedback, if they have too many negatives and mentioning the item they bought was a fake, you might want to be careful. But take a look at the big picture! Let's say they have over 2 thousand feedback's in a year and they only have like 1-3 negatives saying they are fake. Are you honestly going to take the word of 1-3 people? What about the other 1,997 happy customers- are they all that stupid? Not everyone is as skilled as you're going to be at spotting fake frames and may jump the gun! Look at the feedback ratio and go from there.
I hope this guide has brought out a bit of your inner nerd and offered some education so that you, too, can be a fake-detecting, imitation sensing eyewear nerd!
Keep rockin' those shades,