Also see: other methods if figuring out stepper motor wiring
Rodney McLatchy says: "...attach A+ and A- to 2 probes of the scope common to ground, and spin the shaft by hand. On the screen they should come out 180degee. out of phase. If one phase is backwards they would look the same. Or if I mixed an A with a B, 90degees out."
See also:
NEW! Roman Black's Linistep 1. stepper controller kit Proceeds support PICList.com