First of all make sure you do have a real ground, as in these cases most probably they only added a three prong receptacles wothout the third wire.
And even if the ground wire is there, follow it all the way... Since i have seen that groubd line cabled up to the main panel, then dangling unconnected so many times...
Is this a home or an apartment? '50s apartment buildings most probably don't even have a common ground at all.