There probably is some kind of guide to setting the trimpot, but Doddy's first suggestion is probably the simplest and most reliable, short of finding the official guide.

IIRC modern stepper motors are good to over 80deg, but that's only after several hours of running. If they get too hot to touch in under 30 minutes, or noticeably warming up within a few minutes, I'd be reducing the current, as heating that quickly is a sign you're heading to overheating territory.

You could wire an ammeter in series with one motor coil, then pulse the step input until the reading peaks, and adjust to set the current. But that's dependant on the driver not having a reduced output mode after a period of inactivity, and the ammeter having enough smoothing to read the average current without being affected by the PWM output. With the motor stationary, and if you know the motor coil voltage rating, you could also do it by measuring the voltage over the coil, but again you'd need enough smoothing to get an accurate reading, but that could be done with an extra resistor and capacitor to create a basic RC filter.