You can count the right side pedal revolutions for 15 seconds (multiply by 4) or 20 seconds (x3)

But that's just to give you a general idea of your cadence. This flat road steady cadence is often a lot different than climbing or going all-out.

I still use my cadence sensor after 12 years. I have a pretty good idea of a good cadence for me, but it does remind me to shift at least a couple times on every ride.