you need to find someplace where you can see the wiring...attic or basement usually works well. if you see conduit, and NOT loose wires then you've got conduit.
You then need to actually test the ground to make sure the conduit is continuous. You can do that with a continuity tester or multimeter.
You may also see some communications wires, which don't need to be in conduit. The power lines will be black and white running alongside each other.
edit: I should mention for everyone else, those three prong adapters are supposed to be used ONLY with grounded outlets. The metal tab goes into the screw on the faceplate to complete the circuit...but that screw must be grounded. You can test it with a multimeter.
Found some more that will help you test for conduit...scroll down about a third of the way. Very nice explanations with photos.
http://images.google.com/imgres?imgu...a%3DN%26um%3D1