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Test Equipment Recommendations for Measuring USB Cable Resistance
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switchabl:
I think the test current is really the elephant in the room here. There has been some discussion about contact resistance issues at low currents but the resistance will change at high currents as well. The cable will heat up and copper has a temperature coefficient of about 0.4%/K. Well, there goes your error budget.

So what does that mean? First and foremost that you need to make a decision about your desired test conditions. This will also inform your choice of test setup and equipment. The USB-IF specifies the allowed voltage drop in the cable assembly at maximum rated VBUS current. This makes a lot of sense because that is usually the scenario you are worried about in actual use. Depending on the cable that could be as low as 500mA (USB 2.0) and as high as 5A (100W USB-PD).

A normal multimeter will not do 500mA (let alone 5A). You can definitely get SMUs that do 5A (and probably some specialized milliohm meters as well) but maybe not within your budget. A more budget-friendly solution could be a power supply with accurate current readback (e.g. used Agilent 66332A is readily available and does ± 0.5mA ± 0.3%) along with a multimeter for measuring the voltage drop. Really any proper bench multimeter would do, even a cheap old 34401A (DMM6500 is nice if you want a more modern one).

Of course, you are not doing actual compliance testing, so you are free to measure at low current if that is what you care about. In that case a DMM6500 in 4-wire mode might be all you need.

BTW the USB specification also includes "representative receptables" in the measurement along with the cable assembly. In that case, the fixture could simply be a USB connector on a PCB along with 4x banana jacks connected to the power/GND pins in a Kelvin configuration. You might want to compare different connector models to figure out exactly what is a "representative receptable" (or buy a USB-IF approved test fixture). You also need to test the repeatability of the connection. Your 2% error budget may turn out to be too optimistic.
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