Detecting transient voltages in a wall wart
If I were going to build a detector device such as you propose I would spend some time to make it really useful. As such I think you should consider the following list of features.


Have a fast voltage spike detector that detects spikes in the > 6 volt range. 
Have a fast voltage spike detector that detects spikes that go negative more than -1V.
Equip the circuit as a pass through device - charger input - USB cable output to device.
Provide an output clamp circuit that clips any spikes on the output to voltages 5.6V.
Provide a negative clamp circuit that clips any negative spikes on the output to -0.6V or less. 
Provide an LED that lights when there is either a positive or negative spike detected.


Additional nice features if you could see fit to equip the device with an MCU.


Monitor the charging current through a current shunt and switch off for short or overload current conditions. 
Monitor voltage to make sure that it stays nicely within a range of 4.75 to 5.25V.
Add a small LCD display to show unit status such as voltage, current, spike detect count and a charge time counter.