A trick which I learned a long time ago from the writings of Barbara Toohey and June Bierman is to always test the diet soda you have been served to insure that it is really what you think it is. During my travels, I have been served, on numerous occaissions, sugar sweetened soda which was represented to be Diet. I have been served Diet soda when Regular was specified (This could be a disaster if you are trying to treat a hypoglycemic reaction), and I have been served sugar sweetened Sprite from a pitcher in a restaurant that the waiter thought contained water. I have been served regular soda from a tap which although currently connected to the correct stuff was accidentally connected to the wrong tank a little before I got to the restaurant and so was serving regular soda due to the syrup remaining in the plumbing!
I have, in fact, seen almost every trick that one can play with beverage and sweetner combinations. The only way I knew that any of these had occurred was the fact that I always tested the stuff they served me before I drank it. I personally recommend that any diabetic who orders any beverage other than water consistantly test the beverages they are served. Here's how I do it.
While Urine test strips and test tapes are not recommended for use in controlling your blood sugar, they can be used very effectively for testing beverages. All you have to do is to dip the test strip into a sample of the beverage momentarily and watch the color for about a minute. If the strip changes color strongly, you can be sure that you have a sugar sweetened beverage on your hands. If it does not change color at all, you may assume that there are no appreciable amounts of simple sugars present. If the color changes weakly, you are dealing with a system which has a questionable history of isolation between the diet and regular product - USE CARE!
While it was still available, I prefered Lilly TESTAPE for this purpose. It came in a handy pocket dispenser, and one could use a small strip of the tape to do the testing and could therefore get a couple of hundred tests out of it. When they discontinued this product, I shifted to Miles DIASTICKS. This product, while more expensive, did the same job. I would cut them down to a shorter length and then carry them in my shirt pocket in a recycled ACCUCHECK EASY test strip cannister. Very simple, and very effective. Try it. You'll be amazed at what you see.