Detect Selected Radio Input in JavaScript [Mootools]

A collegue brought up an interesting subject today at work; how do you detect which radio button out of set has been selected in JavaScript? This is the first time in a few years I've been stumped on a JavaScript question, so I was quite annoyed! The last time I used a radio button must have been on a MySpace voting pole widget back in the day. So I had my challenge, and I was determined to find out.