The crew has a "hold over" chart that they go by. The de-ice/anti-ice crew will measure the specific gravity of the de-ice/anti-ice fluid. That is combined with the type of fluid used, the out side temperature and that is given to the aircrew.
The hold over chart for the specific type fluid will be broken down in graphs with outside temp and what the moisture is..sleet, freezing rain, light snow, heavy snow...etc.
Also, "de-icing" an airplane is actually a two step process...first it is de-iced, all snow/ice is removed with hot water. Then a layer of heated anit-ice fluid is applied to the aircraft to keep any moisture from re-freezing on the aircraft for a short while. Most airline GOM state that one of the pilots have to visually inspect the wings prior to take off.
De-icing an aircraft is expensive... the lowest charge I saw for my L1011 was $8,000USD and the highest was in YQX at $18,000USD.
Regards,
Greg