Having seen many threads in which there are Earth faults created by bad connections such as
Corroded Earth termination
Loose Earth connection
Missing Earth
Loose Neutral Connection
Missing Neutral

I have been thinking about making an alarm when there is an Earth/Neutral fault.
Bearing in mind that this is to be a permanently connected device, could one measure a voltage between these two points, and if it raises above a certain value, then one could consider that it is a fault.
The question is, at what minimum voltage between Neutral and Earth does one consider to be a fault, or would there be a voltage difference between the two if there was an issue with the Earth connection?