I've had my LT tested multiple times by taking blood and using a device to measure the lactate in my blood. Everytime the corresponding HR has been the same as if I was to simply go out and ride a half hour TT and take my avg. HR. I suggest Friel's Training Bible, its only like 20 bucks and will answer any questions you'll ever have.
I'd second that. If you are at the level where you are interested in LT testing, go out and read about it.
Put simply. Warm up for 5 minutes, then start your HRM recording. Do a flat 30 minute TT at the highest rate you can sustain evenly. The average HR on this ride is your LT. Rememer to warm down for 5 minutes at the end...!!!