Your motors should have some markings on them, but sometimes its hard to identify coz there are lots of versions of exact same looking motor, sometimes even bipolar and unipolar look identical and even have nearly similar codes / identification marks.
Alternatively, just to be sure you can use a multimeter to measure at least the coil resistance, e.g. if its 2-3-5 ohms its ok, if its 10-12 ohms or more then at 12v and approx 1A the rising edge will be slow coz it gets near the ohms law which is the final target and that takes a long time to reach. The rise edge is a function of time which rises fast at first, after which it flattens and in the end needs a long time to reach target. So it makes sense to use it in its first interval when it rises fast, so coils can energize fast and motor can produce power fast. The variables that can be changed for different motors are coil resistance and inductance, these make the time constant, so its good to have motors with low coil resistance, it makes the final value further away and then the usable part of function is in the interval when it rises fast.
Ppls use wordings like "high inductance" or "low voltage" or else, but i think it can be understood better as "high current", e.g. 1-2A instead of 0.25A or better as "low coil resistance" instead, coz inductance only matters for time constant, while resistance also gives the value at which the function aims at (aims at ohm's law given current tho wont reach it, just aims at that value). The voltage written on the motor is nothing but the ohms law = curent rating of wires * coil resistance. The maximum voltage that can be actually used depends on the wire insulation, e.g. 400v or so, and in practice a motor rated at 12v if you try use it at 12v, you would have to wait like an impractical period of time to actually reach the rated current. The current will be cut off coz of frequency and then the power given by the motor will be very much under its rated torque because the current didnt had enough time to rise to a value high enough.
Thats the theory. Practically you could try overcome this with a 24V supply instead of 12V, but its more likely that you wont really need it. You will probably settle for those motors and 12v, since you have them. Under certain work regime, it may give somewhat less torque, but most probably will be enough for reprap. Good luck and have fun !