In short, there is no way to know the humidity without a hygrometer of some kind. The reason why temperature is so important to humidity is that humidity is respective to the temperature in the room, which is why it is known as 'relative' humidity. When the air is hotter, it is able to hold more water in it, so you will find that the amount of moisture in the air will vary immensely from 70-80 degrees, even though the percentage of humidity remains constant.
http://en.wikipedia.org/wiki/File:Relative_Humidity.png
If you look on this page, it will give you an idea of the varying total saturation of moisture in the air as the temperature changes.