YOu can actually get your LT measured with blood work during controlled effort, but really, it is defined as the average heart rate over a ~1 hour race pace effort. When you think about it, this makes total sense. If you were exceeding your LT, you could not sustain it for an hour. Look up definitions. Some use the actual blood values, but many training-based definitions use the effort sustained for an hour (or some other similar time period).

So, just enter a race that will take you betwen 45 minutes and 1 1/4 hours, set your HR monitor to collect data every 10 seconds or so, race as hard as you can and use the average HR for that time as LT. Push to the point of collapse at the end. If you actually fall off the bike at the end and can't get up, go ahead and use the max HR recorded as 100%. If you can get off the bike but have to kneel on the ground to hold it together for a while, call it 95%.

Your max can only be measured with exceedingly hard effort. There are many guides for doing this on your own, which I have done for running. For skiing, I used the maximum HR I achieved in a 5 KM race where I was a quivering mess at the end, virtually collapsing, and seeing stars. I used that number as 95%. The number I got for running produced a similar feeling, but it was a planned effort rather than during a race.

I've measured my LT a couple times skiing 10-15 K races (yes it takes me 45 min to 1:15 to do that, I'm slow). That number, I feel is very important to know too.

I've never measured it for bicycling since I don't "train" but mostly do it for fitness, recreation, commuting and fun.

I do know that my running measured max and my skiing measured max are signficantly different- about 8 bpm. Cycling would probably be more similar to running since the leg muscles are used (skiing is a quadraped type activity, therefore more taxing on the heart to spread circulation to more places).