We'll start by stating the obvious: If it doesn't say "crocs" at all, anywhere on the shoe, it definately is NOT a pair of Crocs.
Next, I do want to make it known that I am in NO WAY associated with the crocs company. I own every type of Crocs and between myself, my husband and our two children (who have both been in Crocs since they could fit in them) we own between 20-30 pairs. And all of our extended families have fallen in love with Crocs, so we have purchased and seen nearly every type.
Until recently, I have purchased every pair of Crocs I owned from department stores and from the Crocs company, either online or at their actual stores. As I started looking at Crocs on eBay, I saw ones I had NEVER seen before in any type of retail store (online and in person). So, to stop any confusion, I called the Crocs 800 number, and this is what I've learned:
1. There are two types of font used on Crocs. One is the more well-know lower-case "crocs" is rounded letters. The second (typically seen on the Canadian-manufactured Crocs) is all upper-case and looks like Greek themed letters.
2. Crocs are manufactured in the US, China, Canada, Italy and Mexico, although the company Foam Creations is in Canada (the company that actually manufactures the shoes).
3. Crocs have only been around since 2003. If they are older than that, then they are not real.
4. Crocs still make both the Beach and Cayman models. The obvious difference is that the Beach model has a scalloped pattern of raised nubs on the top where the Cayman has a simple band of raised nubs on the top.
5. Crocs are odor-resistant; If you buy a pair that smells like feet, then they are not real.
6. Since there are only 5 years of Crocs in existence, it wasn't hard to find out from Crocs that EVERY PAIR of Crocs on the market say either "crocs" or "CROCS" on the back strap. Sorry, but that's what they told me.
7. There are 3 types of rivets: The first type is the smooth button with the croc painted? on. These are the ones that scratch off pretty easily. The second type is the raised rivet (where you can feel the croc). The third type is the plain black rivet, used primarily on the "special" crocs, such as the RX line. These are only used on the inner rivet, and if it bothers you, you can break the rivet fairly easily and order new rivets (free for a pack of 8) from Crocs and replace the plain ones. The rivets do come in different colors now; for instance the khaki/oatmeal mammoth Crocs have a khaki rivet. Every pair of Crocs, every set of rivets pulled up by Crocs when I called, never once included rivets that had "crocs" or "CROCS" printed on them.
8. Crocs are made of a foam material (Croslite), not plastic or rubber.
9. Crocs do wear out. Early pairs of Crocs sometimes did not have "boulder, co" or the crocs website on them. If a pair does not, then chances are that they've seen their better days and are too worn for re-purchase. This is only true for Crocs that have been worn, and not necessarily true for everyone because toddlers tend to not wear out their shoes before their feet get to big and their are peole who purchased Crocs, threw them in their closet and just never liked them (yes, there are people like that out there).
Just be careful when purchasing Crocs. It is not hard for a person to make an imitation Croc and the most important person to suffer for it is you, the consumer.
On a personal note, I don't believe in the Crocs sizing guideline. I purchased the size 10/11 originally (based on their guide) and they were great at first, but as my feet wore into them, they became WAY TOO BIG!!. So my next pair were an 8/9 and they fit like regular shoes at first, but my foot wore comfortably into them like the 10/11's were when I purchased them. So please keep in mind (especially people who are between sizes or 1/2 sizes) that your Crocs will mold to you feet and will stretch on their own.
Some of this information will change as the company changes and there is a new line already coming out. But based on this information, this is what is from 2003-Jan 2008.