Don't know whether there's an easy way to do this without heavy assumptions about what the actual hyperfine fields are (in relation to local moments).
However, if your sample is a thin film with the dominant magnetic anisotropy being shape anisotropy, then you know that your film saturates at Hext=Msat.
And with Fe (I suppose you're doing Mößbauer spectroscopy on Iron), with measurements in applied field, lines 2 & 5 of the sextett will disappear once the magnetization is fully aligned with the field. Putting these two things together, you can estimate Msat from observing the (relative) intensities of lines 2 & 5 as a function of applied field.