An easier way to get the same answer would be to use a cheater plug in the existing outlet but ground the cheater plug to the center screw in the outlet cover. All cheater plugs will have either a metal tab with a hole in the center of it or a green ground wire with a connection on the end. Those are designed to be connected to the center screw on the outlet plate, creating a connection to the outlet box, itself, through the outlet cover screw. Testing for that with an outlet tester or multimeter will tell you.
Some outlet boxes are grounded by the metal coax through which the wires run but some older ones are not grounded at all. If you're lucky, your boxes will be grounded and you can either use the grounded cheater plugs or replace the outlets with 3-prong ones as Drubin suggested.
Some outlet boxes are grounded by the metal coax through which the wires run but some older ones are not grounded at all. If you're lucky, your boxes will be grounded and you can either use the grounded cheater plugs or replace the outlets with 3-prong ones as Drubin suggested.