Resting heart rate on its own is not a good indicator of fitness. Yes, many elite athletes will have low resting heart rates but you can also have a low heart rate for other reasons. Actually if you do have an unusually low resting heart rate its worth checking with your GP. I had a routine health check a few years back where they did an ECG, the nurse doing it expressed concern with my 50 something BPM heart and referred me to a GP, just as well she didn’t check it in the morning as its usually in the mid forties and I am no elite athlete!
I also have a huge heart rate range 45-195 BPM, even though I am pushing 50 but again its no indicator of fitness, its just how I am built.
Some fitness watches will measure your heart rate variability. HRV is is the variation in time between successive heart beats. You would think the more the intervals between beats varied the less fit you would be but the opposite is true, the more variation you see in the time between beats the fitter you are supposed to be. Apparently its an indicator of the stress your heart is under. Does it work… no idea, google heart rate variation and make your own mind up.