And this is why multimeters / voltage testes were invented.
OK, it's not the real reason but you get the point. If you are building your own rig and swapping out PCIe power supply cables, you probably have a tester around someplace and know how to use it.
There are several brands that are plug compatible but wired differently.
There are even
models within brands that are plug compatible but wired differently I'm looking at you corsair...
-Dave