I knew someone who insisted porcelain tasted of salt! but i've always thought you could tell by putting water on the back and seeing if and how quick the water absorbs into the tile. Some ceramic tiles are like a sponge and will take upto 15% of their own weight in water. Porcelain has improved as the manufacturers compete and some now takes less than 0.05% of its own weight in water.
Thanks wetdec, I'm glad its not just me. The strange looks I've had from customers. If you put your tongue on the back of the tile, if it grabs your tongue, its ceramic