Download either EMC2 (Linux) or Mach3 (Windows) and use these to drive the amplifiers used to drive the stepper motors. You can then calibrate a movement driving a nut up a screw thread by a set number of turns and back the same number - do this a number of times and see if you lose registration i.e. the nut migrates along the thread instead of returning to the initial possition. HOWEVER stepper motors need a load to operate properly as resonance can cause the rotor to hover between steps on the magnetic springiness inherent in this type of motor. IMO this is why you experienced a drop in speed against drive voltage. Be-aware that microstepping will reduce the torque available at all speeds and that you need torque to accelerate the machine from rest as well as to maintain the speed.
It is normal to either size the motors by copying and existing known good design or by calculating how much torque is required and the maximum rpm needed along with the step accuracy (microstepping reduces torque so gearing might be better depending on your design) required picking a motor that is adequate in size from the manufacturers data sheet. Unlike a simple DC motor steppers have a falling torque versus speed curve which means that acceleration is the prime factor limiting the size of motor for an application.
Your found motors may be identifiable by their type number or then may be specials in which case you need to experiment to determine their speed torque characteristics. The resistance of the windings and their inductance are good indicators of the torque but there are other factors. The higher voltages are used to help force current into the winding against the self inductance of the winding at the higher switching speeds. The voltage maximum is limited by the insulation of the winding and the voltage the electronics can take safely.
Both EMC2 and Mach3 have manuals which cover the setting up of a typical machine which would help you get going. Measuring torque of a rotating motor requires a dynamometer to be constructed. Not difficult as it is only a pulley with a brake band instead of a drive belt with the motor mounted on a hinge so that the reaction (torque) can be measured using a spring balance only problem is the heat generated. Try using one of the other motors as a generator and load the windings with resistors but take care as the voltage generated can deliver a nasty shock! You adjust the resistors to load the motor. Place the resistors in a bucket of water to dissipate the heat and use the computer program to generate accurately known RPM / PPS. This will tell you all you need to know!
Regards - Pat
Hope this helps