OK...
Apart from getting your LT measured in a sports lab, you can work on perceived effort.

Get yourself on a trainer, on a steady hill that challenges you, or on a flat bit of road in a big gear and go as hard as your can.

When you get to the point of your legs aching with a burning sensation you are entering your lactate threshold. When you get to the point of wanting to throw up, you are almost at your max heart rate.

The zone you want to work in to be working in your LT is after the legs start to burn but before you want to throw up.

Here are a few links that talk about LT on bikes - they may have some useful bitys in or prompt questions you could post in this thread as well...

(There is also some discussion about LT when running too but I am not sure where those threads are.)

Heart rate question
http://forums.teamestrogen.com/showthread.php?t=13036
http://forums.teamestrogen.com/showthread.php?t=17449

Training heart rate zone
http://forums.teamestrogen.com/showthread.php?t=13612

Training for Time Trials
http://forums.teamestrogen.com/showthread.php?t=12489

How to start training
http://forums.teamestrogen.com/showthread.php?t=14660

Faster?
http://forums.teamestrogen.com/showthread.php?t=16427

What is maximum heart rate?
http://forums.teamestrogen.com/showthread.php?t=16963