Hey, good idea - and one I had not considered before! I usually have my wife taste test if I'm not sure, but I do remember accidentally grabbing a liter bottle of regular Pepsi instead of diet once. I got through about half the bottle before my wife pointed out that it wasn't diet! The labels looked exactly the same, except that the diet was just a shade or two lighter blue at the time.
Once I made the switch to diet, I drank an awful lot of it. It was kind of my escape from all the other changes I had to make - my freebie. I drink much less now, but when I'm starting to feel a little pinched by the diet, I'll go stock up on several different flavors to give myself the illusion of variety.

My favorites are (all diet of course) Dr. Pepper, Pepsi w/ Lime, Wild Cherry Pepsi, Cherry Vanilla Dr. Pepper, A&W Root Beer, and every now and then, Coke Zero.
Thanks for the tip!