Use a heart rate monitor for the most accurate data. I personally track my heart rate zone on my Garmin Venu 3s. During Zone 2 cardio, we want our heart rates to be 60-27% of our maximum heart rate, or just below the anaerobic training zone. To calculate your max heart rate, subtract your age from 220.  If you don't have a heart rate monitor available, you can perform the "talk test". You should be able to hold a conversation during Zone 2 cardio.