A brisk walk or a slow jog is sufficient exercise to lose weight. Above this rate you are into your anaerobic level which aids in competitive sport training.
Most athletes use a heart rate monitor to accurately measure their heart rate when training.
You can measure your own heart rate simply, by placing three fingers on the side of your neck and feeling your pulse through your carotid artery. If you count the number of pulses in 10 seconds and multiply that by 6 you will have an idea of your present heart rate. If you combine moderate exercise with a healthy diet and reduce your fat intake chances are you will lose weight, stay healthy and live longer.