Any kind of simple cyclocomputer will do, and strictly speaking, any watch will do.

Set the cyclocomputer so it displays either elapsed time or real time (with seconds).

Count the number of full revolutions you do. For example, count the number of downstrokes by the right pedal in 30 seconds, then multiply by 2.
On a more irregular course, count for 15 seconds and multiply by 4.

With a cyclocomputer, you will also be able to witness your instant speed. It's nice to see that when you travel at 45-50 rpm, your speed will be 15 km/h, but that when you spin at 80-90 rpm, your speed will be 25 km/h, without any more physical effort (so it seems). Call this instant gratification!

Measure first your typical RPM, then, if it's low, focus on increasing your RPM by 10. Then keep that level for a few weeks and try another increase.

Generally speaking, 60 rpm is considered an absolute minimum in terms of long-term comfort and efficiency. Cyclotourists are likely to spin at 70-90 rpm, while racers will go in the low 100s (more tiring but faster). These RPMs are typical values on the flats, but everyone is a bit different and hills, off-road and other "strange" conditions might influence the value.

Once you have an "efficient" and comfortable spin, focus on keeping the same speed under difficult conditions. For instance, when there is a headwind, lower your gear -- use your lowest if necessary -- so you can spin at the same RPM! Spinning at 80 rpm when towing the child trailer in a strong headwind (like I did for 40 km last Sunday) is not really more tiring. It just takes more time and it is a bit more discouraging, but at least, one doesn't get sore knees, feet and leg muscles...

Regards