You can ask them to type one of the following sentences:

"I know what I'm doing, and giving a random website access to my USB host is the right thing to do."

"I'm an idiot."

I love this because the idiots would type out that they know what they're doing and the pros would save time by typing "I'm an idiot."

hah I did think of the second one, but the first didn't occur to me.