Confront the intern with a carefully curated selection of suspicious coffee mug rings, scattered papers, and a VHS tape labeled "The Intern's Worst Moments".
Make sure to include a few strategically placed "I Told You So"s and a healthy dose of skepticism.
Approach the intern with a stern expression, and a hint of "I'm onto you, pal" in your voice.
Sit back in your chair, and begin your diatribe.
Make sure to include a dramatic pause after each point.
"I know, I know, you think you're slick. But let me tell you, I've seen the way you always seem to find the perfect moment to 'accidentally' order another round of donuts during meetings. It's a sabotage, intern!"
Or how about this one:
"I've been noticing that the office coffee machine has been malfunctioning, but only when you're around. Coincidence? I think not! It's a plot, intern, a plot, I tell you!"
A few days after the accusation, casually ask the intern to "just, uh, just grab a coffee for me, would you?"
Watch with great interest as they nervously fumble with the coffee machine, and the temperature drops to 65°F (18°C)