So the outside of the plug should be connected to the white cable, which in turn should be connected to the -V of the powersupply
so normally putting one probe on the outside (sleeve of the connector) and the other at the end of the white cable gives me a reading on resistance
putting one probe on the inside of the connector and the other at the end of the white cable gives no reading
?
Technically it should be "ground", not "-v". There is such a thing as negative voltage. But, if there are only two options: "+" and "-", then "-" is correct.
You are testing for "continuity". So, when you say "no reading", you have to be more specific. If the probes are touching nothing, then what is the "reading". That should also be "no reading", or no "continuity". If you touch the inside (which should be +) to the white (which should be ground), and you get no change to the reading, then that means there is no continuity, or that they are not connected, which is what you want.
If what you mean by "no reading" is that you are getting zero ohms, then that means there is continuity, and that is what you do not want.
I test continuity first - to make sure the cable is constructed the way I think it should be (the component is correct). Then I attach it to the PSU (but not the miner), and test voltage at the plug. That confirms the "system" is correct (did not wire it incorrectly to the PSU).