There is capacitance between the windings and the core/shaft/laminations.
When it is is the open circuit state you can use this characteristic to help narrow down where the open circuit is.
Measure the capacitance of each ring to the shaft.
The ratio of the capacitances will give some guide as to how close to one end the open circuit is.
If you don't have a capacitance meter, apply say 24vAC in series with an AC current meter, on the most sensitive range and see what you get.
You are are applying the voltage between each slip ring and the shaft.
Not ring to ring.
Using a signal generator at between 1 and 10KHz and an old sensitive analogue multimeter (if you can find one) on AC current should work fine.
Some low cost digital meters might give you misleading results.