There are dozens of ways of monitoring how hard you're working during a workout. Prior to getting to that, though, it's important to highlight why this monitoring is important: to facilitate variability of intensity of your workouts over time. Each week, it's important to have easy, medium and harder workouts. It's definitely not necessary or desirable to make every session a high intensity session. Bringing in easier sessions allows the body essential time to rejuvenate and the mind space to enjoy the feeling of movement without having to go to max (worth looking at my earlier blog on HIIT for more on this).
You can use many different systems to monitor intensity including watts, joules, METS, heart rate (in numerous different ways), lactate measurements and SPO2, Amongst the systems, rate of perceived exertion (RPE) is perhaps the most adaptable. Essentially , this involves you using a scale in your head between 0 and 10. The former would be the equivalent of no intensity and the latter maximal. The key is that RPE is specific to a time, place and event or mode of exercise. This is key to understand since folks often think that 8 on RPE means you're working at 80% heart rate or 80% of maximum wattage. This is very misleading. 8 means that you feel that you're working at 8/10 at that time, in that place (think about the effect of the music playing, the atmosphere, whether you've just rehyrdrated) in that specific activity. The heart rate is simply how many beats per minute your heart is beating.
To take this line of argument on a little, it is perfectly possible that you could be working at 5 or 6/10 whilst at a heart rate that is much higher than 80% of heart rate max. Imagine, for example, a marathon runner who's only at mile 5. They may feel pretty fresh and okay. Imagine their heart rate stayed the same, but take their RPE again at mile 15. It'll probably be higher than before even though the heart shows no change. Why? The perception of the same heart rate is that it's harder since they are more fatigued. Max, clearly therefore, doesn't necessarily mean you're sprinting. It means you're going as fast as you can at that time. It may, in reality, be pretty slow.
Imagine that a cyclist is in the marathon. Imagine, also, that they are the same age as the marathon runner from above. It's perfectly possible that the two could be running at the same heart rate. The cyclist might well feel that this same heart rate feels harder than the runner (because the cyclist is much less used to the specific demands of running). RPE ,and its use, has many nuances. I use it a lot. It is highly adaptable, can be used anywhere and gives you an indication of how hard you're working.
It's important, though, to cross-reference it with other markers. Heart rate and RPE are excellent together. Heart rate is not especially useful for short intervals (because of the delay in registering a change); however, it's really good for indicating rate of recovery. The best link, however, is probably with Watts or split time. These provide very precise zones of training; heart rate is normally used as a reference as is RPE. More on Watts and split times in later blogs.