you can count or you can get a bike computer with a cadence sensor - obviously that's easiest. If you can get to the point where pedaling at about 80 feels natural you won't need a bike computer to tell you how fast you're pedaling anymore.
If you're pedaling so fast your butt is bouncing on the seat - it's too fast.
If you're pedaling so slow you are rocking from side to side to generate the necessary power to move the pedals, you're pedaling WAY too slow.
60 rpm is one full revolution per second, so just look at your watch for 6 seconds while pedaling and multiply by 10. (don't crash.
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If your hips rock during a normal, easy pedal stroke then your seat is probably too high. The heel on pedal test is a reasonable approximation of your correct seat height, at least as a starting point.