Count bouth feet every stroke for 30 sec. and that is your RPM
Or do what I do look at my Vetta cycle computer. Sped, distance, cadence, hart rate, all there
I'm 51 here, Western PA has quite a few nasty hills so at times I drop below on a hard climb but whenever possible shoot for 85 to 90. I also found that if you train with a particular rhythm and calculate the cadence, it becomes so natural you fall right back into the same cadence without counting. I would guess everyone has their own "best" cadence, and at that cadence is your best performance.