| I can tell the difference between the two if I have both of them side by side and taste both of them. Individually though, I can't.
I don't press onto the serving staff that I'm diabetic and HAVE to have diet cola but I think sometimes I should. However, I've got some workarounds for the problem.
1. If you're ordering a Coke at the bar, they usually use one of those soda guns with the buttons on them - they press the button to choose the appropriate drink. Diet cola is ALWAYS on the second row - if you see them press the larger top button, it's regular Coke.
2. If you're not sure about your drink and you're out with friends who have regular Coke, ask to have a sip of theirs for comparison. If you can taste a difference between them, then you've got diet. Well, hopefully. It might be that they've got diet and you don't.
3. I think you can actually put Coke on a BG test strip to see if it's got sugar in it.
4.(The 'Never Fail' Test) Tip some of the Coke on your hand and let it dry off. If your hand goes sticky, then it's regular Coke. Diet Coke doesn't go sticky because it doesn't have any sugar in it.
I've found that usually most places are pretty good about getting your order right. But you're absolutely right, nothing screws up your control quite like a non-diet softdrink. I think it's something like every can of coke has about 10 tablespoons of sugar in it or some similarly horrific amount.
Actually I've found one of the best ways to make sure you get a suitable drink is to order beer!  | 
