I believe the OP was asking how forecasters know which to forecast.
specifically to do with the amount of moisture present and the T/DP relationship or is it more than that?
That's a good starting point. Cloud base can be determined by the difference in surface temperature and dew point, they have different lapse rates. Coverage will be a function of instability and dew point. The amount and intensity of rain fall depends on the available precipitable water and the type of clouds expected to develop. After that I don't know.