| If you have a metal box connected to the fuse box by either metal conduit or flex, it would be much better to rely on it providing an independent path to ground than using the neutral. Best way to check to see if the box has a good ground is to measure the voltage between it and the hot wire. Should be the same as hot to neutral. If the box is grounded, you can install a 3 prong outlet, or use a properly installed 2 prong adapter, putting the little tab under the cover plate screw.
The ground wire connected to the screw is a newer, more secure system. For many years 3 prong outlets were just screwed to metal boxes grounded through metal conduit or flex. Conduit and flex can come loose, or corrode breaking the ground path. Both plastic boxes and conduit are common now. They require a separate ground wire. Lack of a wire, doesn't mean no ground. A metal box and conduit doesn't mean it is grounded. There is enough room for doubt that the outlet checkers are a common item. | 
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