Although nothing is more accurate than knowing your true metabolic rate, fitness heart rate monitors that are accompanied by a chest strap are a great baseline for your workouts. You can see how your calorie burn compares on different days depending on the intensity and your stress levels. I also love that so many cardio machines are polar heart rate ready, so as soon as you step on, the machine will begin displaying information straight from your monitor.
Here is a chart for target heart rate zones: Click here
It is also very important to understand your target heart rate depending on your goal in the gym. Did you know that the fat burn zone is actually low intensity? To keep this in check, having a monitor is useful.
I have the Polar FT 40: Click here
Here is a video reviewing the device and showing how it works: