The concept of air humidity is slightly more difficult to grasp then temperature - while anyone can tell, whether it is 10 degrees Celsius or 30C, the difference between 50% and 70% humidity is somewhat more elusive. Fortunately, all you need to know is to keep it between 60 and 80% during vegetative period (closer to 80% for the seedlings), lowering it to some 40-50% at the second half of the flowering period.
The device that measures humidity is called hygrometer and comes either in digital or analogue form (looking a bit like a speedometer in a car). Since the usual room humidity is close to 40-50%, you might need a humidifier to raise it to 80% - though in small, amateur setups, a sprinkler with hot water few times a day will do. In bigger grow areas, a humidifier with a built in hygrometer would be perfect, thinking and measuring the humidity for you.
If your plantation is set in a damp place, think about installing a dehumidifier - a device that will bring the humidity to a lower, specified level. Too high humidity, especially at the later stages of flowering, poses a risk of mould eating into your marijuana crops.