Your thread title is for a 'current sensor' which I assume you're wanting to confirm current flow in the target application and feed this information back into the Raspberry. A 230vac relay across the switched supply will only confirm that it should be working. If this is good enough but you don't want to use a relay consider using a mains neon and a photo diode/transistor to sense when the neon is powered. This will also ensure isolation from lethal voltages.
If sensing the current is really what you want consider a small transformer, ideally a toroid, that you can wrap one or more turns around the core using the wire you want to current sense. You need to be very sure that the voltage in the chosen sense winding isn't too high for your input to the Pi. A resistor or two and a carefully chosen Zener diode should be in there somewhere. Remembering also that it'll be AC and the current in the sensing circuit will be proportional to the sensed circuit current so some sort of fiddle factor could be necessary.
Remember that supply voltages are usually lethal when you shake hands with it and don't underestimate what damage current can wreak on the unwary.
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