A novel method for measurement of the adatom density of segregating dopant atoms is suggested. The determination of the surface density is completely based on post growth concentration profile measurements. The dependancies of the surface segregation ratio on different growth parameters can be extracted. The method was tested on the segregation of boron in silicon (100) molecular beam epitaxy. The strong decay of segregation with decreasing temperature was confirmed and quantified for a selected set of parameters. The boron segregation was shown to be also strongly dependant on doping concentration.