How to detect high current
Allegro has a number of current sensor ICs, based on Hall effect sensors. The ACS712 can detect currents up to 50 A.



The ACS712ELCTR-20A-T has a sensitivity of 100 mV/A, so you can use the microcontroller's ADC to detect when the 500 mV (5 A) threshold is reached, or better, use a comparator, which interrupts the microcontroller. Many AVRs have a comparator on-chip, with an interrupt exclusively assigned to it.

The ACS712 has a current sense path resistance of only 1.2 m&Omega;, so even at 15 A it will only dissipate 270 mW, which it can sustain forever. That's the main advantage over a more traditional current sense resistor as in Rocketmagnet's answer. There you need a relatively high resistance to get the high level at 15 A. Mike calculated that Rocketmagnet's sense resistor will dissipate 36 W when the motor stalls, so timing is critical there (for a moment disregarding the 131 W dissipation in the motor). Even so, a 5 W type is recommended for the sense resistor.