If your steppers and motor are perfoming properly, I’d think it should be starting to get difficult to have them skip by holding them, but I haven’t tried with mine.
If it seems like that’s all working, I’d try turning them up further. The driver is 2A max and the steppers are rated for 1.7A, so with 0.1R sense resistors that would be around 1.4V. Running them at that for a little bit might should work fine, the steppers should get warm and will get pretty hot eventually, the heat-sink on the stepper driver should get warm as well but they should stay rotating and not jitter or cut in/out. They should be pretty difficult to stop at that point, I’d think, as a 40Ncm stepper should be exerting 4kg/9lb of force tangentially on a 20cm/0.8" pulley.