Unless your pre-amp/processor or receiver has the ability to accept a digital (coax or Toslink optical) connection, analog would be the only answer.

Another factor would be the quality of your cd players onboard DAC versus your pre/pro or receiver. Or perhaps to throw in a 3rd option- a separate outboard DAC.

What components are you dealing with?

My mistake for making assumptions. You mean digital input, correct?

What receiver do you have? What CD player do you have?

By careful listening and comparison. Trust your ears. I evaluate components using a combination of
A-B comparison, using reference recordings (music that is well recorded which I know very well) for critical evaluation along with semi-critical to casual listening over a period of time.

High quality DACs will have a subtle audible impact. But the DAC in your CD player may well be "better". Again, what is the hardware in question?

I have a NAD C542 as my transport (a decent player in the ~ $500 range) and a Musical Fidelity X-DACV3 (a 24bit, 192K upsampling DAC that is well regarded).