Just google it...
You can use resistor and differential amp. Or hall current sensor, current monitor digital sensor...
Advantage of hall sensor is that doesn't have voltage drop across it.
Just pick what ever fit you budget, it would be enough to get detection of slave device.
http://www.ti.com/lsds/ti/amplifiers...-products.page
www.google.rs/search?q=current+monitor
etc...