Most good cardio workouts will give you target heart rate zones to hit during your routine. IE 5 minutes at 125 beats per minute + 1 minute at 160 beats per minute.
Most people these days have watches to help monitor, but in case you ever find yourself without the help of technology here's the quickest and easiest way to do it:
1. Place your index and third fingers to the left side of your neck slightly below your chin. Or, to check at your wrist, place the two fingers between the bone and tendon on the thumb side of your wrist.
2. Once you feel your pulse, count the number of beat in 15 seconds.
3. Multiply that number by 4 to calculate beats per minute.
And there you have it!