Stepper motor voltage rating
Steppers don't really have a "voltage rating", it's more a power/torque rating (which is based on the resistance of the coils and how much it will heat up in operation - and it's duty cycle - and then, what ambient temperature it will be in!) Generally, the smaller a stepper is, the less voltage is used, to keep the heat at a reasonable level. But this is not always true, and some "odd" steppers exist.

You could simply drive it from a controller and variable bench power supply at say, 5v for awhile, and record the temperature increase above ambient. In general, most electric motors are spec'd for operation up to ambient + 60°C for continuous use. So if you tested this particular motor, and found the temperature rise to be 20°C, then the volts could be pushed a little higher, perhaps 6v, 6.5v, even 7v, which will also increase the available torque. 

If you find that 6.5v is the maximum this particular motor will run at continuously, but you still don't have enough torque, then there are two options. Either get a bigger motor, or reduce the duty cycle (how often it is "on" versus "off") and continue to increase the volts. For the longest service life, keep the maximum temperature rise as low as possible.