In two-pipe steam installations, steam flows from the boiler to the radiators through one pipe. It then condenses and returns to the boiler through a different, second, pipe. Two-pipe systems are often easy to distinguish from a one-pipe system because there are two pipes connected to the radiator and no steam vent on the side (but not always).
One-pipe systems are different than two-pipe systems in that they usually have a single, large-bore pipe that both delivers steam to the radiators and condensate to the boiler. One-pipe steam radiators always have a steam vent on one side, usually about halfway up their height.