how do I tell digital cables from any other?
Digital transfers involve more than just digital cables. The output device (in your case MD) must be capable of providing a digital signal. Likewise, the input device (your PC) must be capable of receiving a digital signal. There are multiple formats available for digital input and output. The most common digital input and output for MD is optical (though some output coax).
If your MD even has a digital output, it's probably optical. If your soundcard has digital in/output, it may be optical, coax, or both. But I'm guessing if you're running into the mic-in on your PC, it's the built-in mic-in and there is no digital input available unless you buy a new/additional soundcard.
Your best bet is to check the MD and soundcard documentation.
God, I sound like a moron, don't I? LOL
Not at all, just a newbie!