Maybe it's just me, but I've been drinking diet soda for so long that I can taste the difference immediately when I'm served a regular instead of diet (and it's happened quite a few times!).
OTOH, I've been known to mistake someone else's Beam and Coke for my iced tea..talk about a shock to your taste buds!
The idea of testing with a glucose test strip is a good one, too (I believe Dr. B recommends that in his book).