How would you tell if a RCA cord had been plugged in? In this case it would be transmitting audio.
Some RCA jacks are “switched”, meaning they have an extra contact and a spring switch inside the header. If the circuit closes, there is jack inserted.
You can also try to measure the impedance of the line, but doing so without sacrificing audio quality is harder.
I noticed that the SparkFun breakout board had a spring pin. Is this good for any cord?