How to find out if electrical outlet ground is working?
I use a digital voltmeter and measure three ways.


line to neutral shows 120VAC (in Canada)
line to earth shows 120VAC since neutral is earthed some distance away
neutral to earth shows a very low voltage (not always zero, but very small)


If the earth is open, you won't see the line-to-earth voltage on the meter.

This is essentially replicating the functionality of the circuit tester gadget described elsewhere in the thread.