The characteristic impedance of an S/PDIF cable needs to be in the order of 75R. You cannot reasonably measure this, unless you have an infinite length to work with.
Fortunately the impedance of any normal piece of coax will be about right.
So it's extremely unlikely to matter which cable you use for digital, even if they're somewaht different.
But to be paranoid I would take a look inside the plugs and see if you can spot the odd one out.
FWIW if you're serious about S/PDIF connections you use 75R BNC plugs/sockets. But in practice any phono-phono works.