Jules, for an outlet box to be grounded there has to be a "ground conductor" attached to the box. This ground conductor must be normally non current carrying and is usually bare, or has a green insulating cover. The Black wire is the supply side (hot) and the white is the current carrying return wire that is grounded at the panel. The easiest way to tell if a box is grounded is to turn off the power to the outlet, test to make sure it is off, and remove the receptacle. Look into the box and see if you see a green insulated or bare conductor in the cable coming into the box. Many older homes are wired with two conductor cables ( black-white ) and there is no grounding conductor. Another thought...if the cables are routed through a space where they are visible, you may be able to find a label on one to see if it contains a "G" for ground conductor.