There are basically 3 different kinds of methods to measure your LT heart rate.
You can go to a lab and they will measure it for you most accurately. Most of us will never have this done though
You can also do tests at home or in a gym with a helper and get a pretty accurate result.
Check this article out :
http://www.cruciblefitness.com/etips/bike-lthr.htm
If you do not have a power measuring device (like stationary bike or elliptical trainer or something) you can just use the preceived effort, speed and your breathing.
One thing I did for my own test was that instead of using a preceived exertion scale of 6-20 I just did a 1-10. At the end of the test I converted the 1-10 over to the 6-20 scale. The reason for this is that when you are about to roll over and die during the test it will be very hard to think in 6-20.
One more thing that the article does not mention but is helpful is that typically people cannot do more than 5 "iterations" after they reach their LTHR. So if you did a test with increasing power/speed every minute and you lasted say 25 minutes, you have a pretty good chance that you passed your LTHR somewhere between 20-25 mins.