The cheap and simple way is with 25 watt 250 volt lamps.
Take 6 lamps and pair them up in groups of 2.
Series each group then attach one group to each phase . On the other side of the groups
tie those wires together and take them to a normally closed pushbutton. That will act as a lamp test switch ,the other side of the switch connect to ground .
When every thing is normal all 6 lights will glow dim .When there is a ground the grounded phase lamps will go out or very dim and the other lamps will get bright. It is not hitech but it works very well.