One question I've heard through the years is "How do I know if I am working hard enough?"
Exercise intensity is an important part of aerobic exercise and without the use of fancy equipment, there are several ways to monitor the target heart rate or intensity. I think the best for all of us is the use of a heart rate monitor. If it's really important to you, for less than $100.00 this is best way to determine exercise intensity. Another method is monitoring by the rate of perceived exertion. You might have seen the Borg scale charts in your gym. You have to give it a little thought. The Borg Scale starts at 6 and means nothing, no intensity at all. I tell my participants a 6 is as if they are watching boring TV at home. I coach my older adults to work at no more than a level 13 which means the exercise is somewhat hard. The opposite end of 6 is 20 meaning "I can't keep going!" The talk test is the measure I used most often in my years of teaching group aerobic classes. Research has shown it to be a reliable method to determine moderate activity. It is the rate you can exercise and talk comfortably while still speaking 3-5 words per breath. The key words here are the fastest rate. Without someone keeping you on track, it might be easy to relax your workout and not get what you intended.
Resource: Monitoring Your Exercise Intensity, Copy and Share by Brad A. Roy, Ph.D. FACSM,FACHE