How to create a plug detect of a 3.5mm audio jack with 2 connection that carry an AC signal
Realistically, your simplest solution will be to use a jack with a detection switch.

However, since you want an AC signal and your current transformer presumably contains a continuous conductive path through the secondary, you do have the option of using a small DC bias for detection.

Basically, put a large resistor from one side of the transformer's potential connection to a low supply voltage.  Ground the other side.

Monitor the voltage on the side with the resistor, to see if it is being pulled up towards the supply (nothing connected) or if it is being held down around ground (transformer plugged in) and varying with the AC signal.

Tap off the desired signal with a large non-polarized series capacitor which will remove the DC bias - though in actuality you almost don't need that as the bias voltage will be minimal when the transformer is connected, given the likely low impedance of the coil at DC.

You want to make the resistor large enough that your DC bias current does not even remotely saturate the transformer core, or a cause a problem if something else is plugged in instead.  Probably at least 10K, maybe larger - it needs to be many times the DC impedance of the transformer's secondary.

You want to keep the bias supply voltage low enough to be safe, but high enough to be clearly distinguished from the excursion of the AC signal, of course you can low pass filter your detection circuit.