Electronic – Locating pairs on an LVDS cable

First, note that we're talking impedance, rather than resistance, so you can't just put an ohm meter on it to check.

Differential impedance will depend to some degree on frequency, so it's necessary to measure at the frequency which you expect to encounter, and preferably higher in order to give some safety margin.

The only way to test the cable is to send an LVDS signal through it and monitor the received signal. Make sure the receiver is properly terminated, and use a signal at the same data rate which you will be using.

You could probably go the same route that Power over Ethernet (PoE) goes: Have the supply voltage be a DC offset on pairs of the cable, and couple in your receiver through transformers with a center tap on the biased side.

5m sounds a bit much for LV differential, but that really depends on your equalizing abilities.

Also, at some point, the overhead you're incurring by extensive cabling and dedicated differential transceivers might be more than it's worth – assuming your application could withstand the latency, have you looked into using one microcontroller on each end of your 5m distance, translating the unidirectional signals, clock and chip select to a bidirectional bus? I'm thinking about a really minimal system based on USB (that's the cheapest long-distance bus that comes to mind). Or, CAN, which is actually designed for this kind of problems. For both, there's cost-efficient MCUs that come with integrated PHYs.