How to find out if someone who claims not to speak Russian really can speak Russian

Take the Stroop Test: Look at the 1st set of words and say aloud the color of the letters that spell out each word. Now look at the 2nd set of words and say aloud the color of the letters that spell out each word. It takes longer for most people to complete the 2nd task, because the words spell out a different color than the actual color of the words. (This image is from Wikipedia.)

I’ve known about the Stroop Test for long time, and I expect many people are familiar with it. But here’s an interesting use of the Stroop Test that I learned about last night while reading an interesting book called Willpower: Rediscovering the Greatest Human Strength, by Roy F. Baumeister and John Tierney: “the Stroop task became a tool for American intelligence officials during the cold war. A covert agent could claim not to speak Russian, but he’d take longer to answer correctly when looking at Russian words for colors.”

Wikipedia has an article about a related test called the emotional Stroop test: “depressed participants will be slower to say the color of depressing words rather than non-depressing words. Non-clinical subjects have also been shown to name the color of an emotional word (e.g., ‘war’, ‘cancer’, ‘kill’) slower than naming the color of a neutral word (e.g., ‘clock’, ‘lift’, ‘windy’).”