The heart rate monitors on the machines often don't work properly so I wouldn't rely on them. Sometimes they can't read my heart rate at all, sometimes they tell me my heart rate is so slow I must be in a coma, and sometimes they tell me my towel has a heart rate that is above my maximum heart rate LOL.
If you want to know what your heart rate is while you are exercising, your own heart rate monitor would probably be a good investment. If you don't want to make this investment, here are links to two articles on how to tell if you are working hard enough without knowing your heart rate:
Talk Test: http://exercise.about.com/od/cardiow...getheart_7.htm
Perceived Exertion Test: http://exercise.about.com/cs/fitness...ceivedexer.htm
And while I'm posting links, here is one to an article that talks about how intense your cardio workouts should be: http://exercise.about.com/cs/cardiow.../aa022601a.htm
. There are lots of different opinions about this so I'm sure you could find an article somewhere that contradicts everything in this article. I've pretty much given up trying to figure it out. Personally I'm always at about a 7 on the Perceived Exertion Test when I do cardio. When I work out with my trainer, I'm usually at a 7 and once in awhile an 8. I don't seem to have done any damage to myself and I've certainly lost weight.