I knew by the foam.
In a sealed cup, I could tell by the color and character of the foam pressed up against the inside of the plastic lid that she had poured me a regular Coke and not the Diet Coke that my very life practically depended upon. Honest to god, I knew it. I knew! I very nearly said something to her as she handed the plastic cup to me, but I was sure she would think me a nutcase. (Me: "Excuse me, but I believe that's a regular Coke." Her, blinking, thinking to herself: he can't see it; he can't smell it; he hasn't tasted it; he didn't watch me pour it; it's carmel-colored and fizzy; WTF?! But in a Lizzie-like gesture she would say: "Why don't you taste it first, because I'm sure I poured it from the right spigot." And then, like all seers and visionaries, I would have to shame her with my preternatural gifts.) But no. I shot off a flight attendant smile and thanked her and drove away--but slowly, so that I'd be able to catch the drive-thru loop again--undressed the straw in my much-practiced, Fonzie-like one-handed technique, and took a pre-grimacing sip even as I pulled on the wheel to return directly to the scene of the crime.
She wasn't mortified exactly as my bald head shone back into view (I was mortified for her), but she was clearly flummoxed. Turns out, she did pour it from the right spigot, but the manager, who happened to be walking by as I drove around and back up to the window, explained how we could both be right: she said that someone had hooked up a Coke syrup container to the Diet Coke line earlier this morning, but they hadn't bled the lines out completely!
And you all thought I was just a pathetic hack.