As described here (for example), in a correctly-wired receptacle, there will be a small potential difference between the neutral and ground wires that increases slightly with increasing load. I'm thinking that with a bootleg ground, this voltage will always be 0 no matter how much current is running through the circuit. Is my reasoning correct? If so, is measuring the neutral-to-ground voltage a good way to test for the presence of a bootleg ground?
Backstory: I'm in the middle of buying a house in the US. The home inspector discovered that some of the three-pronged outlets had no ground wire. The listing claimed recent electrical upgrades, so I asked the seller to rewire these. I meant for this to be the starting point of a negotiation, but somewhat to my surprise, he agreed to do it. In the spirit of finding a cloud for every silver lining, I'm concerned this means he's planning to do something like a bootleg ground to fool me and the inspector into thinking the problem is solved, and I'm trying to come up with a way to test for this without buying a $300 circuit analyzer.
Backstory update: It turns out the guy just replaced all the ungrounded outlets with GFCI boxes. Which is not exactly what I asked for, but good enough.