I have a stepper motor equipped with what I presume to be a Hall-effect rotary encoder (would that make it a servo, then?).
Since it's a quite old device, spec sheet is unavailable from the manufacturer.
From what I can observe, there's a dipole magnet coupled to the motor, and a set of three strips of wires above it, crossing at 60° angles above the center of the magnet. On the output side, the encoder has 4 pins, two of which being, I suppose, current & ground.
I'd like to read the signal from the encoder. Is there any typical method for that, or does it vary largely from one manufacturer to the other?
You are basically describing a Brushless DC motor, not a stepper motor. The Brushless DC motor requires very special control circuitry that uses the Hall sensors to know where the motor is so the controller can figure out which drive coil to fire. (A BLDC motor controller is required)
A stepper motor usually offers some minor resistance while turning the shaft manually and usually has no feedback mechanism. Is the shaft of your motor practically "free-wheeling"?
I may be mistaken, but I strongly believe the motor is a stepper, not a brushless DC. It does offer this minor resistance you're describing. Furthermore, there is a "step motor" mention conveniently glued on one of its face, along with something about 400 steps per revolution...
The sensor (encoder?) strapped to the motor has a separate wiring: that's 4 connectors for the motor + 4 for the sensor.
If the encoder is on the shaft of the motor, it is most likely a (possibly infrared) LED and photo transistor pair looking at each other through a perforated disc. They sense the holes in the disc as the motor spins.
If it's around the case of the motor then, it's probably a hall effect sensor.
In either case, the number of pulses you get out of the sensor should be the same or an even multiple or division of the steps per rev spec.
4 wires usually indicate a + and - input and a + and - output. Look at the colors of the wires. They should provide a hint as to their polarity. Then test across all pair combinations with a multimeter to get an idea of what each may be doing.