Pressure is defined as force per m^(2). So you can just get a piston of known size (say, 1 cm^(2)), check the force pushing on it, divide the force by size and then convert as you wish.

Also, strictly speaking kg/cm^(2) isn't a physically correct unit of pressure, it's implied that weight in normal gravity is used. "1 kg of force" is actually 1 kg * g (free fall acceleration on Earth, ~9.8 m/s^(2)), so 9.8 N of force.