I approximate it by looking at my watch (or computer if it's working) - 60 rpm would be one time around per minute with both legs; 90 would be a 3-count (left-right-left) every second. I thought about figuring out how many seconds it took for me to get a full round ahead, taking that as "(x + 1)/X x 60 " rpm and decided I would crash into a tree trying to do that and would be really embarrassed trying to explain that in the ER. Right now it's only the cadence that works (the other wire broke), and I've learned some of the correlations between, say, the cadence it takes to go 17 mph in Big Gear #4.