Well; it measures your blood lactate level -- the same way a diabetic measures blood sugar levels (using a lancet). http://www.pureendurance.net/inc/sdetail/402

Lance regularly had this type of testing done by "Dr. Evil." So if you don't mind paying a few hundred bucks, and pricking yourself with a lancet occasionally, you can precisely figure out your LT, and chart lactate levels during your training. Remember though, that lactate levels constantly fluctuate, so there may be problems with when to/how to test during training. It would seem that you'd need a coach or someone else willing to obtain the samples from you basically "on the fly."

There's an interesting discussion of how Lance used this type of testing in the excellent book Lance Armstrong's War: One Man's Battle Against Fate, Fame, Love, Death, Scandal, and a Few Other Rivals on the Road to the Tour de France by Daniel Coyle.