Brain Candy: Savory Riddle Pairs Well with Merlot

Welcome back to Brain Candy! It's 12:30 on Wednesday, which is a good time to logic, and a bad time to do all other conceivable things. I took the liberty of rearranging your schedule for maximum riddle time. Where your calendar used to say "important meeting" it now says "Gerdie" in green crayon. You're welcome.

Today's riddle comes from Brainden.com:

There are three switches downstairs. Each corresponds to one of the three light bulbs in the attic. You can turn the switches on and off and leave them in any position.

How would you identify which switch corresponds to which light bulb, if you are only allowed one trip upstairs?

*Beware of the comments, there be spoilers*