Short circuit protecting a class AB amplifier
Generally you have small (but physically large) emitter resistors, like 0.2 ohm, which I believe set the idle current when both output devices are turned on at the same time:

 source

The TA7317 is an older IC for protecting amplifier outputs and speakers against a variety of failures.  It looks as though it measures overcurrent (shorting) by measuring the voltage across these resistors.  It also detects overheating, DC on the output, etc.

You can also measure the current going from each supply line to the output devices: Over-current protection in audio amplifiers  Figure out the maximum current you'd expect to see for a nominal load in clipping, and then put your protect threshold above that.

You can also put a fuse in series with the output. :)