As you can tell without a computer telling cadence stroke is difficult - too much work. If you don't want to buy a computer with cadence do you have a friend with one? If so pace behind the friend and ask them to count off their cadence. Match them stroke per stroke and get a feel for what its like to ride with a 80 - 90 rpm cadence. I pretty much know without looking what I'm doing. With lots of practice it becomes natural.