I'm currently replacing a old Allen Bradley Bandit 3 setup with MACH3 software on a early 80's vertical mill.  I realize a lot of hardware will need to be replaced, but I'm taking it step by step.
This is for a mill for personal use, so budget is limited and time isn't very important.

Step one is to get the encoders to read on on Mach 3 software.

There are rotary encoders on the ends of the motors. They have 6 wires coming out of them.  Twisted pairs red-white, black-white, brown-white.  How can I meter these or make a setup to figure out which ones are which?  Or do I need to take them apart to see where the wires end up going. These were going to the computer board on the control unit.  There is a seperate tach signal that goes to the servos.

I build electronics for a living so I have access to any testing stuff I need. 
Thanks

I think you will find these are resolvers, if so, not compatible with encoder input.
Measure the resistance of each pair, if very low ohms, they are resolvers.
Nosmo

if the wire was twisted in delta, it was not an encoder

for mach 3, you need ttl 5volt encoder, ttl encoder had 5 or 8 wires (8 wire if differentiel)

They have 'encoder' printed on the label.  Not all white wires were hooked up to the CNC motherboard.

Check for continuity in pairs, if they test for around 28ohms in pairs and no continuity between each pair, then it is a resolver.
Nosmo.

It was a few weeks ago that I was poking around, but if I remember correct there was a similar resistance across all wires, not just in pairs.
From what I see an encoder wound have 6 wires, A+, A-, B+, B-, +5V and 0V.

Differential but no marker then.
With no other data you may have to reverse engineer the connections, one you trace the + & - the others can be detected with a D.B. 'scope, or with a meter if turned as slow as you possibly can.

Been a while, but I took the cover off the motor, and it is clearly labeled. V +/- A +/-, B +/-, and M +/-.  So I'm all set. 
Thanks!