Well, can a standard RCA cable carry the digital signal from S/PDIF from my computer to my receiver?
Yes and No.
SPDIF is a 75ohm signal. Not just any generic RCA cable will be able to correctly carry such a signal.
Shorter, higher quality RCA cables designed for video can sometimes "pull this off", but it's just because the shorter link length or higher quality can carry a 75ohm signal.
RCA only specifies the connector, by the way. So there are indeed "RCA" cables designed for SPDIF signal- they are just designed with higher quality conductor able to carry a 75ohm signal reliably. Most generic, cheap, audio RCA cables- no way. Might work for testing purposes, but I wouldn't recommend for permanent use. Get a good quality, 75ohm rated, shielded RCA to RCA cable designed for SPDIF.
Ok, so I won't get good sound quality out of it, but I can use it to test the connection.