How do I measure S/N ratio?

I just finished building an audio pre-amp, and it is quiet, quieter than the
background noise on some CDs, but I'd like to measure the signal to noise
ratio. Do I measure the voltage at max signal, then measure the noise with no
signal, but same input pot position? Short the input RCA connections? Also,
another question, would caps shunting the power supply diodes reduce hum, or
RF type noise? There is some hum at max volume, (120 or 60 hz., I dunno)but
that's with max rotation of the input pot, whereas, 12:00 position is really
loud music. I want to be able to quantify the SNR so tweaking can be done
productively.
Thanks,
Don