One common method would be the talk test. The talk test means that at low to moderate intensity, you should be able to breathe comfortably and rhythmically thoughout the entire workout. At high levels of cardiorespiratory intensity, your breathing will be significantly more strenuous than at rest. You should be able to talk but find it difficult to speak in full snetences. Another way to measure intensity is with the use of a heart rate monitor.
A heart rate monitor is considered to be the most accurate method of measuring pulse rate. If a heart rate monitor is not available then you can manually monitor your heart rate by taking your pulse.