If you have clients who can feel how hard they are working, then RPE or the talk test is an easy way to gauge their effort level. I’ve had only a few clients who can’t feel how hard they’re working – usually a new exerciser who is less self aware or because everything is “hard,” they don’t distinguish between cardiovascular intensity and muscular effort.
Sometimes I don’t tell clients about the talk test. I give myself the listen test. This works even for people who aren’t self aware because I’m observing them rather than them self-reporting.
I ask them open ended questions so that they talk to me. If they can tell me their entire life story, I know that they’re not very challenged cardio-wise. If they’re conversing but breathing harder than normal, then they’re about mid-range in their heart rate target zone. If they can answer me in one or two words before having to breathe, they’re in the lower end of their anaerobic abilities. If they’re sucking wind and can’t talk to me at all, they’re working mostly anaerobically and I don’t keep them there for long.
Much easier than taking a heart-rate.