On the Raffaele De Ritis' Novelties and Wonders blog I found an old video of Kuda Bux, a performer who claimed to have x-ray eyes. He would cover his eyes with putty, cotton wool, and gauze bandages. Then he would challenge people to write any word, in any language, on a blackboard, and he would be able to magically reproduce what they had written.
Kuda Bux claimed it was psychic ability that allowed him to see while blindfolded, and according to Wikipedia his act inspired Roald Dahl to write the short story of Henry Sugar. Of course, it was really just a standard magic trick. The explanation I've heard for the trick is that it's done by means of the "nose peek." Even though the layers of gauze, cotton, and putty might seem like they would prevent Bux from seeing anything at all, he could actually use his facial muscles to adjust the putty upwards, thereby creating a small space at the side of his nose through which he could peek out. The outer layer of gauze would actually conceal this adjustment from the audience.