Use relatively low concentration of MNPs-dispersion. You may try on and off method. I mean irradiate the dispersion for 30 s and give break 30 s to check the dispersion state. You may repeat this until getting a good dispersion. I prefer this method for magnetic nanoparticles due to their magnetic attraction sometime results agglomeration instead of well dispersion.
You can observe it by necked eyes. If the mixture is fully dispersed, then there is not any observable ppt of the dispersed phase. It will be homogeneously distributed through the medium.