Reflectance difference spectroscopy (RDS) was employed for in situ investigation during and upon the molecular beam epitaxy of II-VI compounds with Mn. In heavily p-doped ZnMnTe:N, at low content of magnetic ions and at growth temperature it was possible to observe below and in the band gap region features occurring from intra-Mn transitions. Furthermore, the overgrowth of Mn islands with mismatched materials was investigated. Reproducibility in size and shape of Mn-based nanostructures was achieved by tracing the formation process via RDS.