It's really nothing more than a parlor, really. There are three easy ways to convince your students that you have an uncanny ability to see from the rear of your skull.
I'm So Sneaky! She'll Never Catch Me!
This is when students are giving off obvious, telltale signs that they are in the process or about to do something "wrong". They'll stare intently at you and attempt to show you just how wonderful they really are. They'll be obviously doing the work and might even offer up a response or compliment for you. They're waiting for you to turn around so they can do something. As you're turning around, you'll even see them make a move for whatever it is they were planning. Continue turning around and when your back is completely turned, you can say something like "Really, Johnny? Did you forget that I have eyes in the back of my head?" When you turn to face the student again, they almost always look shocked.
They Don't Know You've Already Seen Them
Sometimes students have lost track of where you are in the room because they are so engaged in whatever bad behavior they are doing. Someone's boyfriend broke up with them and they are furiously writing a note about it to their friend or Tommy thinks Sally is cute, so he's trying to get her number in the middle of class. I breeze right on past them and walk to the front of the room. Odds are, if you wait about two seconds, they are still doing whatever it is you already caught them doing (though they don't know you've seen them). With your back turned, you can say "Tommy, if you want Sally's number, you're going to have to wait until after class to get it." Or whatever activity it was that they were doing. I did this one on Friday and the students were again shocked at my supernatural powers.
Reflective Surfaces Are Your Friend!
I have an entire wall of windows, windows in my door and a television in the front of my room. This is especially helpful if someone was waiting for you to turn your back so they could get up and pass a note. In the television, I'll see the student get up and begin walking toward their friend. Without turning around, I simply say "Is there a reason you're out of your seat, Johnny?" If you turn around quick enough, you might see him frantically attempt to get back to his assigned seat.
So there you have it. Teachers don't really have eyes in the back of their heads. Students are just that obvious when they are attempting to partake in nefarious activities.