Yep, you control it by the surface area that the water is exposed to air in the bator. More surface area, greater relative humidity. You measure it with a hydrometer or through the use of wet bulb/dry bulb readings.
I think i will use a styrofoam cooler, and put a light in. I will put the eggs on wire, and have a dish under them with water. if the humidity gets to high, how do i lower it?
You also need to have high and low ventilation holes -- and you can control humidity somewhat by allowing more air to pass through the vents (uncovering or covering them more).