Detecting an open circuit by sensing current flow using a shunt and comparator
A good cookbook for op-amp designs (Application Note 31 - Op Amp Circuit Collection) can be found here from the former National Semiconductors (now part of Texas Instruments). On the lower part of page 26, you can find a current monitor circuit. It uses a small value sense resistor in series with the load, like your idea, to measure the current flowing through the voltage being dropped across the sense resistor.