How do you make sure you have the right voltage on the trimpots on an A4988 stepper driver?
Generally speaking voltage on stepstick output should be around 1V.
To imagine more or less what the current and what the voltage is, you can think about it in the same way as about water.
The wire is more or less the same as the pipe.
The voltage can be imagined as (sort of) the height from which the water flows but the current can be imagined as an amount of water which flows. To simplify things we assume that all our pipes are closed into circuit and we have pump/battery and we have a motor which is a reverted pump ;) and finally we have our stepstick which is a tap in our model.
So no matter what the height (voltage) is we know that tap (stepstick) will pass some amount (current) of water. We can drive it turning tap or turning a potentiometer on the stepstick PCB.
So we got it. Principles (deadly simplified) are now clear. See here for more details
Getting back to your question.
You have to know what is your stepstick reference voltage. To make sure about that you have to check out resistor(s) next to main black element on stepstick board. There should be R100 or R200 which are very common.
Now you should read data from motor label to know what is proper current for your motor and calculate
voltage = motor_current * 8 * resistance_of_resistor
So now you know what is proper voltage for your motor and stepstick.
You measure voltage between potentiometer and GND (see on the picture)
If you set and connect everything and start printouts you should check motor temperature. Use your finger. If you can touch motor and hold your finger not more than half a second then probably the voltage set on potentiometer is too high (motor can reach 80° Celsius and it's fine but more will shorten its life span) and you should reduce it a bit (reduce by 5/100 V). If you notice that motor growls or barks then your voltage is probably too low and you can increase it by 5/100V.
Too high current will also reduce longevity of stepstick so cool them out with fan.
Please be noticed.
Z-axis motors will usually be not too hot as they work less than X and Y but as they are both connected to one stepstick so they need more current - set higher voltage there.
Here is a reprap.org site to get basic knowledge about stepsticks.