Testing with a visual indication would be relatively simple-could be a simple go/no go above the inlet, and for that matter the technology exists to prevent energizing a spider box/RV or whatever if a ground does not exist. The problem is demand/willingness to pay for the safety features-someone who has firsthand dealt with a tragedy would gladly hit rewind and pay for safety-but that is not an option. The other issue would be the liability the manufacturer of such a device would be subject to.
I am all for safety devices-but the most effective safety device is an informed, observant, diligent operator. Several years ago working in a manufacturing facility that was big on safety, I made the observation that the maintenance department-who routinely worked in energized panels, ran equipment with guards removed, worked on ladders, etc-had the best safety record. Why? Because our jobs demanded that we be aware of what we were doing-when you get comfortable with what you do it is easy to get complacent and just assume everything will be OK.