Must be old sockets. The only way to tell is by turning off the breaker for an area, test it for juice by having a lit lamp on it when you turn it off or a meter, and look at the wire that is hooked up to the socket. If the wire is a three wire with a ground, then you really need to check it with a meter to see if the ground actually goes to a ground or not. In older buildings, sometimes the wiring can be some weird stuff.
If you test it and it does go to ground as opposed to being "open", then you can install a three prong grounded outlet. If you know someone who is an electrician, have them check it for you. This isn't worth burning down your house on a guess.