There are many ways to measure how hard you are working during any given cardio workout. RATING OF PERCEIVED EXERTION (RPE) can be effective if you use it in the correct way. This is very subjective in that you are determining how hard you are working. When we use a heart rate monitor, for example, we get a number that we can put in a chart to see if we are properly challenging ourselves. The short video below from OCTANE FITNESS gives some guidelines to what we should be monitoring. One thing I would add to the list that, in my humble opinion, is the most important point, is to be honest. When we are honest with how hard we are working, we find that the cardio routine is much more beneficial.