The market for Egyptian cotton sheets is rife with frauds and fakes, so if you don't know what you're doing, it's easy to end up buying a product that isn't the level of quality you wanted. With a little bit of know how, it's easy to determine which brands are legitimate and which ones should be left on the shelf.
The biggest indicator of quality with Egyptian cotton sheets is the price. High end Egyptian cotton sheets are going to cost you hundreds of dollars a set.
The next thing is the thread count. The thread count of high quality sheets is usually somewhere between 200 and 400 max. It's not that higher thread counts can't be decent quality, it's just that there is almost always some level of deception going on with higher thread counts. Manufacturers will use double or triple ply threads to get those high thread counts and those threads are typically made of inferior materials. High thread counts, especially those over 1000, are indicative of some level of fraud.
The next thing to look for is what the cotton is labeled as. Just "Egyptian cotton" doesn't work as an indicator of luxury. Extra long staple cotton grown in Egypt is the best there is, so if it's not labeled as "ELS" cotton, it may as well be the same standard cotton grown in the United States. Luxury of the Pharaohs has more information on this.
Vendor is also important. You can't go wrong with purchasing sheets from high end luxury retailers like Nordstrom and Neiman Marcus. These companies won't sell scam products; their decades old reputations are too important to them.