Stepper motor Identification : Bipolar or Unipolar
You have a universal stepper: it can be driven in either a unipolar or bipolar stepper configuration. Here is a diagram of the connections for a 6-wire stepper:



You should measure the resistance between each unique pair of wires -- this will be 15 measurements. Put them in a table like this:

     1    2    3    4    5    6
1    X    R12  R13  R14  R15  R16
2    X    X    R23  R24  R25  R26
3    X    X    X    R34  R35  R36
4    X    X    X    X    R45  R46
5    X    X    X    X    X    R56
6    X    X    X    X    X    X


Measure resistance from wire 1 to wire 2, enter that for R12.
Measure resistance from wire 1 to wire 3, enter that for R13.
...
Measure resistance from wire 2 to wire 3, enter that for R23... until you are done.

You don't need to measure the values in the cells marked X since those values are redundant.

You'll find that there are two center taps on your 6-wire stepper motor, which will have resistance R to the end taps on that coil, and those end taps will have twice the resistance, 2R, to each other.  The other coil will have infinite resistance to all 3 wires on the first coil.  You will have to test to see what the polarity of the motor is, however.

Useful explanation of 6-wire setup from National Instruments:


  If you have six coil wires, then each phase has a center tap wire:
  
  
  The center tap wire should have half the internal resistance of the full phase. The easiest option is to use a multimeter to find the
  two pairs of wires that have the maximum resistance.
  Connect each phase to the amplifier, and ignore the polarity (+ / -) for now. You have a 50 percent chance of guessing right. The motor should rotate, and if it is in the opposite direction, then switch
  either phase A and A- or B and B- (effectively reversing directions).