If there is no label on the stepper motor indicating the required operating voltage and current, you will need to do some checks.
When driven using a chopper driver, the voltage is somewhere around 10 to 15 times higher than what is printed on the motor label.
Check the motor coil inductance to find the maximum voltage that the motor can be ran at using a chopper driver, then divide that by 15 to be safe
when stepping using a non-chopping driver.
To find the max voltage, use the formula Vmax = SQRT(L) x 32
Example:
Measured 3.2 mH coil inductance.
SQRT of 3.2 is 1.788854381999832 x 32 = 57.24334022399462 (or 57.24V)
Divide 57.24 by 15 and you get 3.816V
This is the data from a stepper I have a datasheet for and used for the example above ( U is unipolar, B is bipolar):
M60STH88-3008DF 1.8 DEG U=3.9V/3.0A B=5.46V/2.1A
U - 3.200 mH 1.3 Ohms per coil 57.2V
B - 12.80 mH 2.60 Ohms per coil 114.5V
Dividing the Vmax by 15 works for unipolar mode as you can see above, but not for bipolar. The bipolar result needs to be further divided by 1.4 to reach
what is printed on the label and I don't know why, but I'll bet Spec does.
Anyway, for the current of an unknown stepper motor, it's best to start low and check the heat of the stepper motor body. If it is too hot to hold for more
than a second, it's probably too high and needs to be reduced. I don't remember where I read that, but will see if I can find it again.
EDIT
Some more reading material:
http://www.mycncuk.com/threads/571-Investigating-unknown-stepper-motors
http://www.geckodrive.com/support/step-motor-basics.html