Grab a voltmeter (set to AC auto range or a manual range of at least 125V) and check voltage between ground and neutral - not sure what an acceptable level is numerically, but if you have more than a volt or two, I'd suspect that - there's always some distributed capacitance between the two, but shouldn't be over a volt or two.
Another - but strange, thing to consider, and this is a long shot - those three light polarity testers available from Home Depot (two greens - you're clean!) can be misled. If you swap neutral and hot and someone at some point has wired a phantom ground (tied it to the neutral line), the tester will show the polarity is correct even though the ground is actually hot. Again, not likely in your situation but people have been killed incorrectly renovating K&T (knob & tube) runs.
Bren R.
