It isn’t difficult to tell whether you are working at the right intensity, even without a heart rate monitor. A generally accepted guideline, call the “Talk Test” will indicate your level of intensity and give you a good gauge of whether you need to push yourself harder, or slow down a bit.
When you’re exercising, try carrying on a conversation with someone, or if you’re alone recite something like the Pledge of Allegiance. If you can carry on a light conversation while exercising then you are in a good intensity range. As soon as your speech starts to slow, break, or you are having difficulties you know you’re working too hard.
Researchers at the American College of Sports Medicine have discovered that those who can carry on light conversations are at the appropriate intensity level, even corresponding to exercise prescriptions (like the target heart rate) from doctors or trainers.