what do you mean detect?
if it's 12v DC, just hang NPN transistor from it, referenced to ground (if you use a bjt be sure to use a generous base resistor) - when current is present, the transistor will turn on, no current, transistor off
alternately you could hang an optoisolater from the 12vdc (again, resistor) - current, opto output low, no current, opto output high (assuming a typical inverted opto)
I don't think DC can be sensed from induction, so you'll have to put a shunt in series with the load to actually measure the current.
I2R tells us 2 amps will generate 400mW across a 100mOhm shunt, dropping 200mV - hook that into an op-amp and boost the voltage a bit and you've got a simple ammeter